Many forks for cantilevers have a couple of holes where a caliper brake would go, but if there isn't a usable hole in the fork crown, you can remove the standard fork crown fitting from the mudguard and use zip ties instead.
Standard SKS P35 mudguards.
From the outside. Note the bit of rubber jammed between the mudguard and the underside of the fork crown. The cable ties where holding the mudguard higher than the underside of the brake caliper, with the result that the front end of the mudguard got forced down onto the tyre.
From the inside.
2 pairs of holes either side of the mudguard centreline, made with the reamer bit of my swiss army knife and the hole edges sealed with a spoke heated on the gas.
hob.
If you are using caliper brakes, mudguards are easiest if you replace the allen nut by one of these
nuts from Gilles Berthoud (also available in 9, 22 and 30mm lengths). They tighten up using a regular spanner, and are threaded inside so you can attach the mudguard using a bottle cage bolt or similar. The Problem Solvers Fender Nut is similar.