Not infrequently people will post questions in our community forums (http://libertybasic.conforums.com and http://groups.yahoo.com/group/libertybasic) asking how to implement a programming language. Usually someone will chime in with a link to some very detailed tutorials hundreds of pages long. This is fine if the reader is sufficiently well motivated.
The other day I stumbled on a very cool page on Wikipedia about Tiny BASIC, a language implemented many years ago and published in the seminal Dr. Dobbs Journal magazine. I realized that budding language implementors would really go bonkers for this!
Check out the links at the bottom. There are a couple of implementations there written in BASIC with source code. What better way to get your feet wet but to port Tiny BASIC to Liberty BASIC or to your favorite BASIC? Doing the port shouldn't be too hard, and you'll learn enough to venture adding some new commands. :-)