I always start by sewing the base to the arms. It's a super simple straight stitch. I take my pieces and place them right side down in place and make pins. A few tips on this:
1) Put your pins EXACTLY where you want to sew, starting them where you want your stitch to start and ending them where you want it to end.
2) Trust your pins. If you've placed them where you want to sew, sew exactly where they are. That sounds self-explanatory but after the 12th time you've pinned and removed the cover to sew, you will question your pins. You will be going cross-eyed, y'all.
3) Do not, for the love, pull too hard on your fabric to make your cover crazy tight. By all means, make it fit, but if you make it too tight, you will end up with a cover you can't put back on after you wash it. I made this mistake the first time I made a cover and ended up having to re-do it. Whomp, whomp...
This is the part that's the hardest to explain, because it greatly depends on your couch's shape. You can see the order that I connect my pieces above and copy that if it works for your couch. You will be working with your cover inside out as you pin and sew. After you sew each piece, be sure to look at it right side out to make sure you didn't sew an awkward bunch or make a mistake on the shape.
This is us a few steps in. You can see we have sewn pieces 1-5 in and are working on pining on the front arms. This step is one of the hardest because it involves a tricky curve AND piping. I like to pin the pieces together first and then go back and put the piping in. I just take out a couple pins at a time, put the piping in (make sure the raw edge is facing out and the cord is on the right side of your fabric) right along where my seam will be, and put the pins back in place.
You can see my pins along the shape of the curve here and alllllll the extra fabric I will go back and trim later.
My sister is going back and placing her piping along the seam here (you can see the end of it curled up in the chair waiting to be pinned/trimmed).
One big huge piping tip: make SURE you leave about an inch of piping BEFORE you start and about an inch AFTER you finish. You will need to put this little extra piping through your seam so that it looks finished or it will be tucked under in the couch's hem.
This will make more sense as you sew, but make sure the cut end of your piping is tucked through the back and you sew through it to hold it that way. I had no idea how to finish my piping the first time I made a cover and did realize until I was finished how I should have done it.
After you sew your piping in, the rest is downhill from there. Next, I sew the front of the back (#9). This is where your couch will start to take shape.
The last pieces I always put on are the back pieces. I use two pieces that close with Velcro in the back. I first sew the Velcro on the two pieces and connect the two pieces with the Velcro so that when I pin it to the rest of the couch, I am working with it as one piece.
When you pin the back on, be sure to pull your side pieces to that the couch takes shape but don't pull too hard. The back of my couches was kind of tricky because of the arms. What I ended up doing was folding the fabric around the curve to create pleats.
Once you have finished sewing on all of the pieces, go back and trim your extra fabric. Make sure to place your cover right side out on your couch or chair BEFORE this step. You would hate to find a huge mistake (like the unfinished end of piping poking out the front or a weird bunch in the your fabric) after you've cut the extra off. It's almost impossible to fix without that extra seam allowance.
The final step is to hem your couch. With all of the couches I have done, I have hemmed the edge to fold just under the chair and added Velcro to hold it in place. This isn't absolutely necessary but I think it has given my chairs a finished look and keeps the covers in place.
(this is the underside of our ottoman)
After I fold and pin my hem, I attach a strip of Velcro all around my cover. I then staple the other side of the Velcro to the underside of my couch. It's worked like a charm!
I hate to be a Debbie Downer...even though you feel SO good, you still have a long way to go. The cushions are beasts. Of course you could stop here and leave your couch like this (or at least something like this).
Next, we'll talk cushions. But take heart...that's the last step!
(Psst...miss part 1 and 2? Find them here and here.)