Oh man, last part! The side pillows are probably the easiest, just because they're tiny. I deconstructed the old side pillow covers, to reuse the cording and zippers, and to use the pieces as a pattern for the new fabric. 2 zippers, 2 bits of cording, and the side panels for the pillows.














Covering the cording is pretty straightforward: wrap a 1.5" wide strip of cloth around the cording tightly (right-side-out for once), and stitch it up. I might be mistaken, but I think it's advised to use a special foot for sewing things like zippers and cording. Since my sewing machine is an awesome old 1970s hand-me-down from my former landlady, it doesn't have all the fancy attachments, so I just did it with a foot ordinaire, and it turned out fine:














Cut out two panels per pillow, slightly larger than the pillow itself:














Since there are no edge-panels for these, to accommodate for the pillow's shape, cut a 2-inch slit diagonally inward from each corner, and stitch it together like so:














Putting it all together with the cording is only slightly tricky. Same basic concept as all the others: pin it all together, inside-out and tightly. Working with the cording, you have to feel at it the whole time, to make sure the cord-part of the cording is lined up straight on the seam you're trying to sew. I'll try to illustrate it with a few pictures, maybe it will be more understandable:
If you look closely, you can see that the cording is facing the inside, sandwiched between the two side panels of fabric.











Being stitched up.













Below is the mostly-finished side pillow cover. To accommodate for the zippers on the bottom, hem the bottom edge of the panels:














The couch, in all its glory:














I'm pretty pleased. Notice the attempt at aligning all the patterns across the component pieces. There's only one really badly matched part: it's discontinuous across the left and right seat covers. I definitely had enough fabric to do it correctly, but I took leave of my senses when I was cutting it out.
Secret: it's still not finished-finished. I haven't added the zippers to the bottom of the side pillows yet, mostly because I hate sewing zippers and was in a hurry to finally have a finished-looking project. I'm also going to make the little arm-covers out of the leftover fabric, because the left arm gets a lot of sitting, and I would like to keep it as clean as possible. I probably won't bother posting the how-to for the arm covers, because it's pretty much the easiest thing (next to a handkerchief) you could make.

A quick note on supplies:
A good fabric store should have a reference on hand regarding amounts of fabric necessary. Bring in measurements and/or a photo of your furniture, and if there's anyone helpful working there, there shouldn't be much of a problem figuring it out. Mine, a smallish loveseat, required only 10 yards. Prints and geometric patterns generally take more, because you'll need to match the pattern on all the pieces.
As far as thread goes, go with something pretty heavy-duty (like a polyester) for the top, and then a lighter-weight thread to go through the bobbin, so it doesn't get jammed. It looks like I will be going through about 100 yards of thread total for this project (that is, 100 yards for the top and 100 yards for the bobbin). I could simplify and say 10 yards of thread for every yard of fabric you will be using, but I don't know if that's just a happy coincidence with my sofa or not.

Now for the part you sit on! Seat cushions are not hard to do, but they are time-consuming. One takes about 2 hours, although the other one will probably go more quickly.
The method is basically the same as with everything else: cut out the approximate sizes and pin them inside-out to the pillow you're covering. I used the same double-flap method for this, but made a mistake. I made the double-flapped opening much too short, and had to wrestle the pillow pretty forcefully into the case. Fortunately it is a pillow and not a person. The double-flapped (or zippered if you are ambitious) part should extend around the back corners at least 2 inches, to make for a larger opening and subsequent easier dressing of the couch.
Pictured below are the top and bottom panels, the two too-short hemmed pieces for the opening, and the one strip of cloth that goes all the way around. It's a little tricky getting the pieces sewn together around the double-curve at the piece that sticks out in front of the couch's arm, but not too difficult. It seemed easier to make it look good when I sewed with the long-strip side facing up, as opposed to the large pillow-shaped panel, if that makes sense.














The same technique applies to the cushions as did to the rest of the pieces: pin it inside-out, stitch it once, try it on, and double-stitch it. Below is the once-stitched piece:














And here is the reason why I like to check it after I've sewn once, instead of going ahead and double-stitching it without trying it on:














Oops. An easy fix, but I would've been pissed if I'd gotten it all double-stitched and on there for good, only to have to take it off and fix it so the border didn't show.

The whole pillow, off the couch:














That tricksy little corner, up close:














Here is the much-referenced hemmed opening in the back of the cover. You can use leftover and misaligned pieces for this part, because it will pretty much never be seen.














Oh man, this couch is so close now:














I've done an OK job with aligning the pattern from back pillow to seat pillow to front panel of seat pillow to front panel of base cover, considering it's my first slipcover evar. This is, however, a good opportunity for anyone else to learn from my mistakes and plan it out a little more carefully; it's certainly possible for the whole thing to align perfectly.

After a week of Ugly Sofa Time, I am finally in a position to start covering more pieces. Do not misunderstand, this takes a really long time. The base by itself took about six hours. A good way to break it up is a Sunday for the base, then one pillow a night after work until you're done. A week of transitional sofa is totally OK. Two weeks is borderline unacceptable, so I covered the two back pillows at the same time. While not as labor-intensive as the base, it still took longer than I thought: about 3 hours total.
The pillows are done in basically the same way as the base: take note of all of the separate panels on your existing pillow, and cut pieces out slightly larger than that. I really hate sewing zippers, so to get the covers on and off, I just made a double-flap at the bottom of the pillow where the zipper would be. Being that it's a double-flap, you cut out two pieces for the bottom.














Start out by hemming the bottom-double-flaps:














Then sew together all of the edge-pieces, bottom and sides. Lap the two hemmed bottom pieces over one another in a way that will make sense when it's a pillow cover. The picture probably illustrates that better:














The two hemmed parts are overlapping one another, facing in opposite directions.

Now do exactly like you did with the base of the couch: pin the whole thing together as tightly against the pillow as you can:














Then take it off through the bottom flap and sew it! Double-seams are a good idea here too.
It's worth pointing out that the original back cushions had two buttons on them. I contemplated doing that with the slipcovers too, but it would be really difficult to anchor them well enough to keep them from popping off. Feel free to try it yourself, if you're more ambitious than I.
It's... still an extraordinarily ugly couch, but it's on the way! At least it's more than 50% new upholstry.

I am in the process of slipcovering a seriously homely inherited loveseat, pictured below. After a cursory Google search, I found little more than "Throw a sheet over it and staple it down!" Which is unacceptable. My mom gave me general directions of how my grandmother used to do it, and it's shaping up to be perfectly acceptable, so I felt it appropriate to share with the whole of the internets how I am doing it.














Cover the couch base separately from the cushions, to avoid the slipcover pulling apart from the sofa every time you sit down. This uses a lot more fabric than just tossing it over and stapling around it. The upside is that it looks consistently presentable and is not obnoxious.
To get the fitted base cover, measure and cut out slightly larger than the size of each fabric panel of the base upholstery. Then pin it all together tightly against the couch -- be careful not to pin the couch, because you're just going to have to take the whole thing off and sew it.
As far as the part that goes under the seat cushion goes, don't waste your nice upholstery fabric on it. I cut apart an old pillowcase, and it works pretty well.














Here's a closeup of the arm, all pinned up. The curves are pretty hard to sew, most of mine ended up looking janky.
















After you're all pinned up, it's not a bad idea to take it off and put it on right-side-out, just to make sure your patterns line up and seams match.














Take it back off, turn it inside-out again, and sew. I actually drew on the exact line to sew, because I was really paranoid about screwing it up. It's not a bad idea, just make sure it's inside-out. Don't worry about the hem yet.
If you want, since couches get a fair amount of wear and tear, stitch the whole thing twice. You might as well, you're already sitting there with the sewing machine.
Once you're all sewn up and pins removed, put it back on the sofa inside-out. Trim up any extra fabric around the seams; it'll just make it bunchy and weird if you leave it.
Now for hemming, the easiest part. Flip it up, making sure that your pattern is even and that it stops just above the floor evenly all the way across. Pin it, take it off the couch for at least the third time, and hem it up. Be sure to iron it too, to get a nice crisp edge.














Because I am fastidious, I ironed the slipcover on the couch when I was done, so none of the fold marks from the fabric would show. This is not necessary if you are not crazy.

Ta-da!













The problem with taking on a sofa incrementally is that you are left with an uglier product than the original. Right now, this is pretty much the worst sofa that has ever existed.