Get a zipper / piping foot for your sewing machine if you don't already have one: it will make it so much easier to put those in.
I have one, but it's still a pain in the ass.
Mostly just because it takes the extra time to sew two pieces together, then sew them together again, and so on.
The zippers are for the cushions, which I think work best with zippers. I'm going to use velcro on the slipcover itself. Besides, I don't have to buy the zippers. I'm just ripping them out of the old cushions and reusing them.
I did take before pictures, embarrassing as they are. I actually cleaned the sofa before I started working on the slip cover with my new carpet cleaner's upholstery tool. It did a pretty great job, so I wish I'd taken the before pictures after I did that just so it didn't look so grimy. I've already ripped apart a couple of cushions, though, so that's out of the question.
Did I mention I'm using striped fabric? So far I've managed to work it out, but I do have to admit that I didn't appreciate fully how much work goes into matching stripes.
That said, I've managed to (I think) line up the first cushion with the lines on the slipcover itself, so woohoo for that. Now to do the other three haha.