On a bike with a sidepull brake caliper you would remove the reflector bracket, if present, and secure the light bracket in place between the caliper and the fork crown using the caliper mounting bolt itself.

If you have a different brake setup but your fork crown is drilled as if it could accept a sidepull caliper you would just need to obtain a bolt to secure the light bracket in place.

If no hole exists you will have to figure some other creative way to rig the light.

-j