On a D-reg just order the STC from Petersen
Use of Mogas outside the USA(about 300 euro), they have an order form you have to fill in.
The STC comes with all paper work including the stuff for the LBA(German CAA)
Put the stickers next to the fuel caps.
Some metal data plate has to be fitted to the engine.
Sign off and log entry by a mechanic.
Done.