I just discovered the awesome MBlab project, but I still doesnt understand how clothing works. I read about proxy system, but I don't totally get it? Do I have to model the clothes manually and fit to the models? Or there are basic clothes model available for donwload that work together with these models?
If I recall correctly, you'll likely have to nudge a pre-modeled article of clothing around your character manually. That sounds more intimidating than it is, though, if you're patient, have experience deforming meshes, and are handy with proportional editing.
I think one process could be to give your coat model, pants, shirt, etc, it's own armature with automatic weights and get it 90% of the way just through posing. Bake it out once it's close and then tweak it with proportional editing (if it needs it).