first, look through your commit history, and get the commit you want to merge from the master branch, you should see something like this:
$> git log
[...]
commit c5960dbe4674ae72f80cbe1ed5eb0cc690062c7a
Author: (...)
Date: Mon Nov 18 11:42:58 2013 +0100
this is a commit message!
[...]
now switch to your staging branch and run
$> git merge <commit-id>
If you didn't do anything weird, git should be able to fast forward this, and you're done.