The most used way is to shove a toothpick through the trailer just in front of the hook perpendicular to the hook. I used to use that method but noticed it didn’t allow full motion of the trailer where the body flops up & down when swimming or bottom jigging the jig. So I came up with another idea that allows the best action of a flat, plastic frog-type trailer while doing the least damage to it.
Get some 24g coated wire from most any store that sell craft supplies.
I wrap a piece around the hook’s curve and shape both ends into an L:
NOTE: For illustration only, the wire is shown on top of the trailer, but for changing the trailer the wire should be beneath it with the prongs facing upward.
Hook the trailer’s front and stick the wire into the body:
The other way is for when the trailer is threaded up the hook closer to the jig’s head. I use the same wire except secure with a wire tie.
It rarely slides down but if it does, reattach the wire.