One thing has me somewhat baffled: How do you decided in what order things need to be assembled? Is it only with experience, or is there some rule of thumb?
Great question! And, I don't really have a great answer. I've been sewing stuff for over 30 years so it just comes naturally now but I always approach it the same way. (Thanks high school home-ec class)
1) Draw out the idea. 3 dimensionally helps.
2) Break it down into its component pieces and parts.
3) Create your pattern for each piece.
4) Mark and cut out each piece with alignment cuts/marks and drill holes/alignment marks.
5) If you haven't visualized how the item goes together start putting the pieces/parts together without sewing anything until it creates the end product. (eg: sewing in zipper coils on each piece before sewing the pieces into the larger panel.)
6) Start sewing it together and remember this.......you'll make a mistake and have to rip-out the seam and start over again. You just will and its not a big deal. Once your repeated the assembly process on the same project multiple times you'll no longer make the mistake but, one-off projects and limited experience may result in having to rip-out a seam or two to get it to go together.
30 years in and I still have a sharp seam ripper and xacto knife! Good luck and post up some of your projects.