I have been washing Pottery Barn, Ikea and custom slipcovers for many years and this is how I do it...
Step 1 - While the slipcover is still on the sofa, do any spot treating with a little detergent and a toothbrush if you need to.
Step 2 - Remove the slipcover and wash the part that covers the body of the sofa first. Wash it on cold water for about 20 minutes on the "normal" setting. Longer if the slipcover is really dirty.
Step 3 - Hang the slipcover outside on a clothesline or 2 chairs with the slipcover draped over them.
Step 4 - Repeat above with the cushion covers.
Step 5 - While the slipcover is still a little damp, put it back on and smooth out any wrinkles. You can probably iron it if you feel like it but I never do.
Step 6 - Do the same with the cushion covers.
The whole process is pretty quick, especially if the weather is warm. So now you know.
ps - A couple of more hints. I vacuum the sofa a couple of times a year. It's pretty amazing how dusty they get.
Also, I put a couple of throws on the sofa if we are going to be watching football and eating, as we are slobs.