This video covers the basics of L-System algorithms and how they can be applied to "turtle graphics" drawing in Processing.
(If I reference a link or project and it's not included in this description, please let me know!)
Read along: natureofcode.com/book/chapter-8-fractals/#chapter08_section6
Algorithmic Beauty of Plants: algorithmicbotany.org/papers/#abop
L-Systems in Computational Beauty of Nature: books.google.com/books?id=0aUhuv7fjxMC&pg=PA78