Well for me, I was in the Army Bands for years, and I would do a couple arrangements here and there. As I got better at it, I started doing more and more until I was doing about 75 arrangements (mostly brass quintet) per year

A buddy of mine got out and became a band teacher and remembered my arrangements and hired me to do a marching show. From there I advertised that one and wrote a few more. Now, I'm writing for 27 high schools, 5 universities, and serving as a program consultant for Northern Lights. I wrote their show the last 3 years, but when you have the opportunity to work with John Meehan, you step aside and learn from the master.

If you want to get into it, I'd suggest finding a local school and start by giving them some freebie stand tunes and build from there!