SKS guards are a piece of piss to fit!*
All you need is something decent to cut the stays with
Quick instructions
1) Remove wheel
2) Bolt guard to fork crown. At the rear you may need to drill it for the seat stay bridge, I use a stanley knife though, twisting it to make a nice neat hole (Takes practice)
3) Fit stays to eyelets, point them at the mudguard fittings, tighten
4) Fit plastic things to guards
5) Hold guard where you want it and mark the stay in the middle of the plastic thing, or thereabouts
6) Snip
7) Bolt stays on, uppermost first.
8 ) Adjust mudguard by sliding up/down on stay to get it central to tyre
9) DONE.
*LBS mechanic, I fit loads, practice makes perfect!
and what if you have discs? or want to run a wider tyre?