If there is a mailing list for the module, sign up and send the info there. Usually module maintainers prefer to get changes in the form of a diff file (usually diff -u is the prefered format). If you don't use *nix, and don't have diff, then I'm sure you could send the entire file that you changed.

Also, module maintainers are usually very happy if you include some new tests for the test suite that make sure your new feature is working properly.

Before you go an do all that work though, it might be worthwhile bringing it up on the mailing list to see if the maintainer will be receptive to the change or not.