Ok, you basically have to do this in order to achieve perfect mapping, but you have to experiment a lot until you get used to it.
This is a bit sketchy but it's just basics.
First start creating squares like a dice, 6 of them. (I only did 5 because whatever).
Then you paint, the single squares at the sides will be painted too, then you have to move them to fix all the sides.
After that you have to move them to the main template (use those from above).
And then you are finished, all will be perfectly mapped. Practice will do everything.