It depends on the species fish as to how you use mealworms.
Our Ornate Bichir, Dragon Puffers, Brown Puffers and one of our Fahaka Puffers takes them live and whole off the end of a pair of feeding tongs, our other Fahaka puffer prefers it when they've metmorphosed into the pupal stage, then we just drop them in one by one until he's had enough (about 20)
Our Purple Spotted Gudgeon like to take them whole and alive floating on the surface and our Parrot Cichlilds and Kribs take the drowned ones that sink to the bottom.
We've tried the giant mealworms too, but none of our fish seem to like them.