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.