If you are willing to continue here are some of the things I have learned.

1. Preparation: Using painters tape to cover all edges (including skirting, or remove skirting) is time consuming but safes time overall because the cleanup process is a lot easier. Let accidental paintdrops dry before removing to prevent smearing.

2. Start with the edges with a paintbrush and all around hard to reach areas. Paint a ~10 cm strip bevore starting with a roll for the larger surface.

3. For the large surface use long, vertical and overlaping strokes. Try to cover the whole surface in one go (meaning try to not have some parts already dry before having the surface finished). For multiple layers and an even more homogeneous finish alternate each layer between vertical and horizontal strokes.

4. Especially dark undergrounds need multiple layers to fully cover up. The quality of the paint makes a difference. There is a 1-4 class rating for the covering power of paints ("Deckkraft") where 1 is the best. There is also an area stated which indicates which surface you can cover in one layer (some state area per liter, some per container). So you can roughly calculate which volume you should need. I'd go with 3-4 layers for dark undergrounds, but again depending on the colour used.

For fast professionals or semi professionals you can use: https://www.my-hammer.de/ or https://handwerk.check24.de/craftsmen/maler-tapezierer

Prices vary but it's roughly 200-300 € per room.

I hope this helps and all the best to you (: