It's very very simple.
You take Das klei no matter wich kind (I took the white one).
knead the clay untill it is soft.
Spread it out as a large flat surface about 2 to 3 mm thick.
Cut it in size of the area that you'll cover.
Put it on the model and be sure it lays realistic.
Then be sure it will not fall in, that isn't happen in realtime also, so on your model too, so I made 2 bars over the area.
Then you must wait untill it's dry, then you weather it etc.
Very simple and easy to make, it takes about 10 minutes to make, and a day till it's dry.
Thanks for the comments!