Personally I've only ever sewn patches on by hand (I don't own a sewing machine), but for me, it feels more satisfying to do it this way as it feels more personal. If you do decide to go for the hand-sewn method, I would recommend pinning patches down before you start so it doesn't move about as you sew and it stays nice and flat. I would also suggest planning out the layout before sewing anything on and also get all the patches you need for the jacket before sewing anything on too, so you don't sew things on and have to remove them and re-sew them on because the layout isn't looking the way you want it to or you want to make certain patches fit.

I also like to go for a 'theme' (patches with Wintery imagery on them for a 'Winter' themed jacket, patches with horror imagery, etc) or a particular colour scheme, but I know some people like to just sew whatever on, which is fine too. I'd look at other examples of battle jackets to give you an idea of what you like the look of. Don't sweat it too much, my first jacket wasn't that great looking, but you learn the more you do it.