Hi,
The general workflow is:
1. The first thing to do is to clone the repositories and try to build a distribution (there's some documentation here:
http://ceylon-lang.org/code/contribute/). This is actually quite hard at the moment because there's quite a lot of changes happening right now, so don't be put off if it take a while.
2. Once you've done that you can create a test case alongside the other ceylondoc test cases, which should fail right now.
3. Then you can try to fix the bug, so that the test case passes.