Hi Jochen,
My mudguards are the MuckyNutz Mugguard ones. MugGuard - MTB Mud Guard made from 100% recycled plastic!
Short at the front and long at the rear. I attached them with zip-ties and it was quite simple. I didn't need to modify the guards at all.
I'm really pleased with how they fit and work. The bike still gets muddy in the English winter, but the mudguards help to keep the worst of the mud off my face and the suspension.
I hope this helps. Please let me know if you need more information, or detailed photos of how they fit.
Porcupine