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.