If you just want to take a precompiled binary and install it, you just have to write a derivation wrapper around the binary which will declare the expected hash and take care of e.g. unzipping and moving to $out/bin.

Here is a more involved example of downloading a release and then extracting the binary from a .pkg file: https://gist.github.com/J-Swift/364a8b158bf0b603f6e784e454ca...

Here is a more simplified example: https://gist.github.com/mitchellh/c47e3333bb78f57836ba2aa806...

EDIT: to get the sha, unfortunately you have to perform some esoteric command line incantations: https://github.com/NixOS/nix/issues/1880#issuecomment-366615... and https://gist.github.com/boxofrox/d8a3080fbb03f84b7d7a31e102b...