There are two ways to get a copy of the Rosegarden source code. First, if you have a sourceforge account, it's best to checkout the code so that you can commit changes in the future. For this:
$ svn checkout svn+ssh://email@example.com/p/rosegarden/code/trunk/rosegarden rosegarden-svn
Replace “userid” with your sourceforge userid.
If you don't have a sourceforge account you can get a copy of the code “read-only” like this:
$ svn checkout svn://svn.code.sf.net/p/rosegarden/code/trunk/rosegarden rosegarden-svn
Since this copy isn't associated with a sourceforge user, you will not be able to commit changes from this copy.
Once you have a copy, you can use svn update periodically to keep it up to date:
$ svn update
After making some changes and testing them, you can examine the changes with svn status and diff:
$ svn status $ svn diff
And commit them once you are happy with them:
$ svn commit