Have a look at http://progit.org/book/ch8-1.html
The basic workflow:
# step 1: clone from svn
git svn clone http://svn.silverstripe.com/open/modules/gallery/trunk
# step 2: add a single change from your modified vesion
# add to staging
git add
# commit (to git, not svn)
git commit
# repeat step 2 until all changes are in
# make sure you still got the latest version from svn
git svn rebase
# output all commits as patch files (and file them as a patch ticket on open)
git format-patch
You'll notice there isnt an "git svn dcommmit" or "svn commit" step in there, for the first time we'll review changes, if thats all good we can give you commit access :)