I'm going to be very blunt and very honest.
This cosplay is one of the more popular ones of Tsubasa Sakura. The very first thing you need to do is research others who have made, bought, or had a commission done on this outfit. I promise you'll find the answers you're looking for.
Some really great places to look are cosplay.com (of course), acparadise.com, deviantart.com, and google-images. But you really have to dig deep. I mean, some people will explain in detail how they made it, and others won't explain anything at all. If you have no sewing experience, then ask someone you know who does. They might even be able to help you.
Another thing you might consider doing is looking at the image of the costume, and in a sense, re-draw it on a fashion template. Draw both the front, back, and sides(optional). This will help you get a feel for what the garment might actually seem like. The thing about CLAMP is- they like to defy gravity in their artwork, and many times they won't repeat the same garment.