For the initial prep, you can still go through all the decontamination steps Matt uses, just be careful and test a small section with stuff like iron removers. Use care while claying baring as well.
Check out Matt's 2-part series on how to maintain and coat the vehicle properly.
Detailing a Frozen (Matte) Black M5 - Part 1
Detailing a Frozen (Matte) Black M5 - Part 2
Matte paint isn't anything to be too scared of. You can treat it basically the same as gloss paint; just be careful of scratches. It may not scratch any easier, but you cannot fix scratches through machine polishing. The protection you use will make the biggest difference in keeping it nice and easy to clean. You cannot use a normal wax, but a ceramic coating like Gtechniq Crystal Serum Light or Jescar Power Lock Plus will work great.`