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.
Once I discovered `lib.fakeSha256`, I just put that in the derivation, try to build it and then use the error message to find the correct hash to put into it. Probably not the fastest way to do it, but it's easier for me to remember.
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...