First of all you can run
$ subgit configure --svn-url PROJECT_ROOT repo.git
Then edit repo.git/subgit/config depending on the following condition. If you want continuous synchronization, do you want other branches be ever translated (e.g. when someone pushes refs/heads/new branch, should it be translated to SVN)? If yes, you should have the following configuration:
trunk = trunk:refs/heads/master
branches = x:refs/heads/develop
branches = *:refs/heads/*
#it's up to you whether you want to have tags/shelves or not
#shelves = shelves/*:refs/shelves/*
#tags = tags/*:refs/tags/*
If no, you can specify that certain branch only:
trunk = trunk:refs/heads/master
branches = x:refs/heads/develop
#it's up to you whether you want to have tags/shelves or not
#shelves = shelves/*:refs/shelves/*
#tags = tags/*:refs/tags/*
In the second case when you push refs/heads/branch, SubGit will ignore it.
If you need just one time translation, both configuration are the same.
Finally, run
$ subgit install repo.git
If you don't need continuous synchronization, you can then run
$ subgit uninstall repo.git