I have personally sewn a shirt before. I was bored one summer and my mom signed me up for a sewing class

haha
Anyways, make sure you don't use a sharp needle to sew shirts. If you do, it will tear through the shirt and you will have a bunch of holes where the stitches should be. Instead, use a needle that has a ball point.
You can sew on pockets using shirt material, it just won't be as rigid and abrasion resistant as, lets say, what the Crye Combat Shirt is made out of. You could put a lot of starch on it though, which would make it more rigid if necessary.
You would sew on the pocket before you sew on the sleeve. Make sure to sew on the velcro, then the pocket, and then the flap, which I assume will also have velcro first sewn on it.
I think you could pull off sewing the pocket on a white t shirt as white t shirt material is easy to match, and then dying that t shirt into a color of your choice, but buying a t shirt and finding matching material would be a pain, and sewing an entire t shirt would be a pain also. It was hard enough for myself to sew one, even with an instructor who had been sewing for over 40+ years by my side.
Hope this answers some questions.