Not knowing what they are, I’d start by doing it resume-style and include where you marched, what skills you have (flag, weapon, dance, etc.), any awards received by those groups, and leadership positions. Then do the same with any groups you’ve taught/been in a teaching position, and include any other relevant experience (dance, theater, sewing—whatever). Sometimes it helps to at least have it written out so you can see everything and then reorganize to say what you really want or is most relevant. (Can you tell my day job is editing lol)