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Thank you @cobmaximus!
However I have to admit that it is the way you intitally thought: I've traced/copy the reference by drawing on a layer on top of the reference :/
It is not in the step-by-step animation because otherwiese the gif would have to show much more steps... I am pretty bad at portraying, that's the reason :D

haha well that's fine, is a way to loose the hand and pay more attention to details :)