I have been fender shopping and going crazy. One day I want hammered Velo Orange metal fenders and the next day I want SKS Plastic Longboards. I have read many of the blogs. Some say metal keeps you drier, but others say metal dents and bends and is a mess when you are locking your bike or banging it around in a storage box for transportation etc.
So Plastic or metal that is the question?
A trick I have found that sometimes helps for custom fabricating and mounting fenders is to use cardboard as a shim between the tire and the fender to get the fenderline exactly where you want it before any mounts are made. Use whatever number of layers of the cardboard you have on hand to get the thickness you need as tire clearance. Cut strips of cardboard from a shipping box to be laid over the bias of your tire to shim the fender up and support it every so-many inches around the diameter of the tire. How many spots you need to shim depends on the fender material -you'll need more locations on floppy plastic fenders and can get away with just 3 or so on a steel fender at/near your fender-mounting points.
Loosely wrap some masking tape around the fender and rim to make it just firm enough to keep the fender from moving around as you make your marks for holes and measuring your stays. This way you don't need all the many arms of Shiva to work and measure while holding the fender with the others.
Laying a bit of masking tape on the fender itself to help you mark with a sharpie where a hole is needs to be drilled is a good idea too. You can find the mark easily later and not put any stray marks on a plastic fender. It's almost impossible to make marks on a steel fender that you can find again and you don't want to make a mistake and drill in the wrong spot or even a few mm off. Using tape will show you exactly where your hole needs to be drilled when you have it on your workbench.
Take your time and be prepared to remove the fender and wheel a few times as you go along, especially if you have not done this before. Don't try and drill through the fender while still temporarily mounted on the bike. Take it out of the frame and away from the tire (d'oh!) so you can drill it cleanly and straight-on on your workbench without putting a long curling ugly drill-scratch on the surface away from where you wanted your hole. It's almost impossible to get a drill lined up and perpendicular to the fender while still in the frame. If you try and drill from the inside out you are just guessing where it needs to be. Guessing does not yield perfect results.
Aim small, miss small.