No carbon in the filters and be sure to have extra aeration in the tank, as most medications tend to lessen the oxygen in the water. If the fish are still eating, sprinkle some of the powder on some on their food, then give it to them. Do this during the early stages of treatment, since whatever they expel can then be killed by the Metronidazole in the water so they aren't reinfected. Many discus keepers recommend this type of treatment as a regular routine for all new fish in their quarantine tank. In most cases, it works, but in some, the fish may get worse and eventually die anyway, so keep in mind it is meant only as a last effort to save the fish.
Many drug stores sell syringes made for force feeding baby medication.
If necessary, mash it up a bit to make sure it will pass through the syringe. You'll need to have this mixture at an acceptable temperature for the fish, so you'll want to float the container in some tank water or other warm water. You'll also need some type of tray or dish with 1 or 2 inch sides, in case the fish decides to flop around a little while you're treating it. Suck up your mixture from the cup into the syringe, about 2 ml per fish to be treated. Larger fish can have more like 3-4 ml of the mixture.