Github has a simple web interface that makes it very easy to propose a simple change to code. You do not need to install any software or do anything beside register for a git hub account.
These suggestions apply to forks of both joomla-cms and joomla-platform, although the screenshots are for joomla-cms.
Identify a change you want to make.
Find the file you want to modify on Github
Click Edit to open the file
Make your change
Fill in any additional information in the pull request including a reference to the joomlacode issue.
Click Send pull request.
Make a note in the joomlacode issue with any a link to the pull request.