Hold the assembly as you would an ordinary gun. (If you've never held an ordinary gun, go and watch five minutes of television -- that should give you sufficient gun knowledge to be able to puzzle out how to pull a freaking trigger.)

Now pull the trigger. The top spring arm should snap forward. If it slips, or snaps too easily, further modify the notch, so that it will hold the spring arm without slipping. If the snap action seems weak, the spring may have been weak to begin with, or you may have worn the spring too much working it onto the clothespin. Disassemble another clothespin and be gentler this time when you put the spring on.

When you're satisfied with your trigger assembly, you're ready to finish assembling your gun.