From a howto on Robby on Railshere. You’ll want to read http://www.rubyinside.com/advent2006/12-piston.html for more commands and reasons to go this route.
First get the Piston gem:
Now take your svn externals and convert them to being vendor branches managed by Piston:
If you have been lazy and haven’t updated your externals for awhile, you may need to run “svn up vendor” first or perhaps a lot more to get things in order.
After everything is convert commit your changes to subversion and you are ready to roll with piston: