Trial-and-error mostly. I cut my modding teeth just looking thru the code until it made sense.
I suggest you just focus on what you want to change instead of trying to understand everything. It's just too complex for a full understand of all the code.
As for making DF compatible, if someone is serious about it, what really needs to be done is a lot of beta testing. Hopefully, if the tester just moves all the asset files from the DF mod folder into the OJP folder, they should be able to load up the levels manually and test them for compatibility.
---Jedi Guardian of the Newbie Questions
---Masters of the Force Team Leader / Creator
---Open Jedi Project Lead Moderator / Co-Founder