Worms live at different levels in the soil, some quite near the surface and some digging down 6 feet or more. If you want to be able to gather worms for fishing, to feed a pet reptile or bird or just to see what kind of worms are there, you need a way to bring them up out of the ground. Mustard is one way to accomplish that.
Make a dilute solution of mustard and water. Use about 1/2 cup of mustard to 3 or 4 gallons of water. Stir well so that the solution is thoroughly mixed.
Find an area with plenty of earthworms. You can identify places with active worm populations because the worms leave little piles of castings -- worm excretions -- on the surface by their burrows. Large, open areas are best for finding night crawlers, and the piles of castings are easy to spot on flat ground.
Clear any loose litter from the soil. This includes leaves, loose grass, twigs and anything else that might prevent the mustard solution from getting into the soil. You don't need to scrape down to bare dirt, just remove enough litter that the soil is exposed enough to allow the liquid to reach it.
Pour the mustard and water mixture over the ground. Pour enough on the soil to allow it to sink in, but you don't have to drench the soil with it.
Watch for the worms to come up. They will come to the surface in response to the irritation of the mustard. They may come right away or it may take a little while for them to respond. How long it takes depends on how deep the worms are in the area where you have poured the solution and the soil conditions there.