Everything begins with the enclosure. If you decide to drill the enclosure by yourself, use this template:
As an alternative a drawing of the dimensions:
Depending on the accuracy of your printer you have to modify the template. I use the scaling function of the printer driver. (If your printer tampers the picture horizontal and vertical differently, you need to open the template with inkscape and correct it there)
Before painting grind the enclosure with steel wool. The particles of the wool can be washed away under water.
The enclosure seems to be clean but if you check it with grain alcohol and a paper towel you will see that there’s still much dirt on it.
Just before priming the enclosure I clean it again. This time with break cleaner, so it’s absolutely free from grease.