I thought this would be simple.
The problem is that the entire brake assembly needs to be rotated to screw into a thread built-in to the fork. There's also a hex/allen hollow tube-nut at the back (see red arrows), but this doesn't do anything until the bolt has screwed through.
I was hoping to put the metal loop of the mudguard in front of the fork so the brake would clamp it tight (with a washer not pictured). But when you put the mudguard in place you cannot rotate the brake to screw it in, because the lever arms catch on the plastic mudguard.
Is there some trick to this bicycle-tetris that I'm missing or is this simply the wrong kind of mudguard for this fork/brake?