I found two vintage armchairs, both really needed a new covering and a new padding. Therefor I bought enough fabric (this one from ikea) and new paddings.
Step 1: Add the New Padding
First I took off the legs. Then I cut out the new padding according to the surfaces I wanted to cover: Seats and backs by just sticking them on the old covering with glue. It takes a day until the glue dryes.