@katieisnell for testing, follow some of the examples in the project. Maybe this one for app-root-path that I did recently.
https://github.com/flow-typed/flow-typed/blob/master/definitions/npm/app-root-path_v3.x.x/flow_v0.142.x-/test_app-root-path_v3.x.x.js

Group your tests in describe/it blocks and then write tests that validate your library definitions so that if someone were to accidentally change it to something unexpected it would fail. Remember you are testing just the definitions not the library itself so test if you can use a function in a particular way you expect.

Also don't forget to add some cases you expect to fail and then use suppression codes to validate they fail as expected, this will be helpful if you want to check that function A definitely returns void or something.

Once you've written some tests you can run it with./quick_run_def_tests.sh if you are on linux/macos orquick_run_def_tests.cmd if you are on windows.