This is part one of the building process, since some people's final designs may vary. Just research, designing, and some calculator math in this part. Don't skip this part however, it's pretty important.
Step 1: ¡Research Time!
It's always good to know the details of your project before you start. Google Images is your friend in times like this (or whichever search engine you use). Make sure to find images such as "wing anatomy", types of feathers, the bird you are going to reference, etc. I do suggest Google Images though. that's where I found all of my references. In my case, I am using Barn Owl feathers to build the wings.
Step 2: Ask Around
If you can, try to find other people that have attempted your project in the past, you'll find some useful tips to help you with your own project. I got the idea for my main harness from an Ulquiorra cosplay on DeviantART. The cosplay can be found on her page: http://koltirasrip.deviantart.com/
Also, while you're there, check out her comic... it's pretty cool.
Step 3: Sketch It Out
Try to visualize what the wings will look like on your body when they're finished. Get the general concept down first and then try to see how it will stay on while you're walking around all day or however long you plan on wearing it. If you're making large-scale wings, also try to consider what you will do when you sit down. It's at this point that you should consider what materials you will be using. PVC is a good idea for a frame like this, but a good alternative is thin wood or plywood. Go through different styles to see which one will work best for your cosplay.
Step 4: Sketch It Out
Make a template for your wings. I taped 18 sheets of copy paper together to make a 66 x 25.5 inch paper. I traced as much of myself on the paper that I could and had my mom finish up the rest. Then, I traced the harness concept onto the body. After that, I traced the primary wing section ("outer edge" of the wing) on top of that with an orange highlighter.
Step 5: Ratios!!! (Oh, Come On...look at Least a LITTLE Excited)
I traced the longest part of the orange outline's length and width and turned the paper over. I traced out the area I would need and redrew the rest. At that point, I took the measurements and put them on the sheet of paper. I then took a small ruler and measured the length of each of the three segments of the open wing diagram and solved the ratio for the rest of the segments. (I even have the math to prove it) So now, I have the lengths for the other two segments.
Step 6: Feather Time.
Since I'm starting on the primary feathers first, I had to figure the length of each primary feather. I figured out the length of the longest and shortest primaries, since those are the ones I'm making first. I then figured the length of the coverts, alulas, and the secondaries. The first primary ended up being quite big, as you can see in the last picture above.
Step 7: Final Touch-Ups
I realized recently that a LOT of harness designing had been left out, so I started sketching like crazy. I counted the number of feathers I needed and worked out every mechanic detail. I ended up deciding that I was going to have the wings rotate a little at the base. I'm also going to put a form of stopper so the wings won't just...fall. I also plan to attach each primary feather with a small dowel section for support and hold them together with string as shown in the final image of this step.
Step 8: Prep Work
Now that the designing is completed, we can start on the actual building, and at a close shave too. Considering that I have four other cosplays to finish, one of them being my own, and school hasn't even got out yet (Finals Week, thankfully), I say it's crunch time.