Here are some how-to details from my slipcover project.
Total cost was about $225. This included fabric, new foam seat cushions, Scotchguard and shipping.
For comparision, a Pottery Barn slipcover for almost the identical sofa costs about $800.
For fabric I bought five sets of sailcloth curtains from Target. Each panel cost $10 and measured 42 by 96 inches, so I got fabric at less than $5 a yard.
I'm always down for what's cheap/easy. The other reason I went with curtains is that the edges are already hemmed. I can't cut or sew a straight line, so having one straight edge already built in gave me confidence to try this project. The curtain route also reduced the cost of a potential Craft Fail to proportions I could cope with.
On the down side, the fabric is thin. You can only do this on a sofa that is already white underneath. Even then, some of my seam allowance shows through. (For heavier bleachable white slipcover fabric, I would consider Dharma Trading. Best prices I could find.) There is so much labor involved in this I would advise not to let fabric price be the biggest factor. Go for something durable because believe me you will never want to do this again.
I won't go into detail about how to pin fit a slipcover since many other blogs have covered this. (For some examples, check out Honey Bear Lane or What the Craft.)
The only thing I did different was cut my pieces so that the bottom edge used a pre-sewn curtain hem. Pre-wash and iron the curtains of course.
I loathe the shabby chic look, so I spent a lot of time trying to get the slipcover as tight and fitted as possible. For a more modern look, I skipped welting. I lie. I skipped welting because it's too much work and I don't know how to do it.
To keep the look as modern as possible, I also eliminated the skirt. I raised the hem of the slipcover to show a little bit of the sofa feet. (For some tips on how to modernize a slipcover, visit Naomi at Design Manifest.)
I have a zipper down the back of the slipcover and on all the cushions. For a zipper tutorial, see this post.
Since I live in a 480-square-foot apartment, I had to burn out the bulk of this over one weekend. Our home looked like the mafia searched our place for a micro chip. If you live in a house, I would say haul your sofa to some dark corner and work on it slowly.
Here's the before - not bad, but the seat cushions were bulging and there was dinginess that no amount of cleaning would remove.
Here's the after.