I want to alter a raincoat how do I do it?

It keeps me warm and dry but it's a bit of an eye sore. It's one size to big cause I forgot to look at the size before I got it. Is it possible to have a raincoat takin in?? I also want to add a few things to the outside of it but I don't want it to spoil in the rain. Can anyone please give me a list of things that I can add to the outside of my raincoat that won't get ruined in the rain and cold??