How are those files composed, what are their structs and other info.
All OFP tools listed which help you to edit it.
Mission editing 101.
Making your own models, hints tips and guides.
These are actually called Islands in OFP (but in our wiki to keep standard format we call them terrains).
It is essential that you do addon AND release properly. This guide helps you make good release.
How the locality is handled in multiplayer missions.