git clone https://github.com/big/project.git

• You now have a $GOPATH/src/github.com/big/project directory, which is actually a git repository.
• Fork big project on GitHub, and copy the SSH URL shown for your fork into clipboard.
• Go into $GOPATH/src/github.com/big/project and type:

   git remote add fork git@github.com:JamesClonk/project.git

• Make code changes, compile and test them, then:

   git push fork

• Then you can open a pull request to get your code merged into the main repository.
• To keep your fork in sync with the original repository, you essentially need to do:

   git pull --rebase origin
   git push fork

• Before creating pull requests its probably best to rebase to clean your commit history.