Before you sew
1: Patterns
There are some things you should take into acount before you start sewing a costume, like who you are cosplaying as and how the costume itself is made up. Looking at the different pieces of the costume and how they work together is very important. Sewing from patterns is probably easiest, even if you have to make a few alterations for it to work. For Sheena's shirt, I couldn't find a pattern that would work. I do have a FOLKWEAR Hakama and Kataginu pattern, which helped immensely visualising it, but the pattern just wouldn't work for what I was trying to sew. So another thing you can do is make your own pattern. Making a mock-up of a pattern out of pattern material or cheap fabric is helpful. I had a bolt of a bunch of pattern material lying around, so I went ahead and made a pattern for her shirt. It was made of three different pieces: the back and front of the shirt, and two pieces of neckband that were sewed together.

The dashed lines are where I'll sew. Her shirt is easy because I don't have to fit anything. The obi, or the fabric around her middle that holds everything in place, does that for you. That leads us to another section...
2: Fabrics
The fabrics you use can greatly affect your costume. A linen will be stiff, whereas a knit will be stretchy and drapey. You also want to look at the materials in the fabric. Is it synthetic or natural, like cotton or rayon? Or is it a blend? Synthetics won't let your skin breathe as much, but are sometimes better to use. Color is another thing you want to look for. Matching the exact color and type of fabric is usually hard to do, so don't be overly picky. Try many different fabric stores before you actually buy something. Try to find clearance or fabric that is on sale so you don't spend too much money.
As I work on my costume, I'll add more and more to this section. Check back for updates!
3nodding