how would one go about contributing to such a project?
The exact details depend on the project, but in general:
(1) Visit the project's website.
(2) Download the source code. There should be links on the site for this.
(3) Study the code, start tinkering with it, learn how it works.
(4) Possibly subscribe to the developer's mailing list. Find out what the hot topics are for the project.
(5) Also, study the issue tracker to find out what bugs need fixing.
(6) When you feel comfortable with the workings of the code, start thinking about how you can improve it. Think small to start with. See if you can fix a few bugs.
(7) Submit your fixes using the instructions on the site. This will usually be via a code repository system like SVN.
(8) Hopefully your fixes will be accepted by the project leadership. If not, don't panic -- maybe you missed something? Maybe you didn't stick to their coding style? Whatever, talk it through with them, find out the problem, and try again. (and if you do get accepted first time, don't let it go to your head!)
(9) Congratulations you have now contributed to an OSS project.