When I first read In Fire Forged, I really liked it, but saw things I could improve. So, I left some high-quality reviews on fanfiction.net (that is, reviews that demonstrated I somewhat knew what I was talking about) and then solicited the author. From there, networking (people who you collaborated with can collaborate with you).

Back-engineering, I'd tentatively suggest just posting somewhere with reasonable visibility that selects for writers you'd like to collaborate as, and then ask anyone interested to ping you. Alternatively, you could develop a relationship working on someone else's writing and then ask them to look at your's.