Assuming you've already installed Crystal, you can follow this very simple guide (note that it does break any changes you make to the serve directory in SVN):
$ cd path/to/crystal/checkout $ cd serve/ $ rm * -Rf && rm .svn -Rf $ svn co http://svn.gna.org/svn/bongo/experiments/avocado ./
Now we can start the server!
$ cd ../ $ ./run-crystal.sh
Great! We're done! Head over to http://localhost:8080/ and awe in the awesomeness of Crystal and Avocado working together. :)