Fork ymasory/sbt.g8 on GitHub to your account.
Let's assume your account is "foo".
Clone it.
$ git clone git@github.com:foo/sbt.g8.git
Now make your desired changes.
Do a local deploy of your modified template and try it out.
$ cd sbt.g8
$ rm -rf target
$ sbt
> g8-test # must result in SUCCESS>exit
$ cd target/sbt-test/default-*/scripted
$ java -jar target/*-assembly-*.jar # must print "hello sbt.g8"
If you like your new template, push it to GitHub.
$ cd sbt.g8
$ git push
You can now access your modified template using g8.
$ cd
$ g8 foo/sbt
If you'd like to share your changes, send a pull request.