I've been out of the loop with Nix for a little while now, but I was maintaining roughly a half dozen packages for a little while (including semi-complex u-boot stuff), and I learned everything I needed to know from this tutorial.
I would second this. Learning nix takes time and practice, but is very rewarding. Reading the nix pills series is definitely the most helpful first step. I would also recommend https://discourse.nixos.org/ where kind fellow Nix users constantly answer questions and give advice.