You need to do this only once. The instructions here are very similar to the instructions at http://help.github.com/forking/ — please see that page for more detail. We’re repeating some of it here just to give the specifics for the scikit-image project, and to suggest some default names.
If you don’t have a github account, go to the github page, and make one.
You then need to configure your account to allow write access — see
Generating SSH keys help on github help.