Simply clone the repository with the hgsubversion
extension enabled, and chances are you're good to go.
There are, however, a few...
Things you should pay attention to
Non-standard SVN repo layout
You might have some tweaking to do if you don't have the classic trunk / branches / tags layout.
Large SVN repo (lots of commits)
The initial clone is time consuming, and it isn't an operation you can resume should the connection with the repo go down, meaning that all the work will be rolled back.
The solution here is to only clone the first revision with
hg clone -r1 svn://repo
Then you can
hg pull
which you can resume, and will pull all of the other commits.
Misc
Once the clone is complete, adjust the [paths]
section of .hg/hgrc
to make it point to the right repo, or get rid of it if your clone is going to act as the central repo.