It's not difficult at all to fish a nymph (or any sub-surface fly). You still want a drag-free drift, and you want the fly to travel just off of the bottom (even ticking rocks from time to time). At the end of the drift, let the line hang in the current for a few seconds before you begin to strip the line back in. This simulates a nymph emerging from the bottom to the surface. Many strikes occur as the fly naturally rises from the bottom at the end of a drift.
I find that a pinch-float works well as a strike indicator. The trick is to set the pinch float at a point on the line that allows the current to carry the fly just off of the bottom. If the current is strong, the pinch float will have to be set higher up on your leader.
In strong currents, a small split shot twelve inches from the fly will help to keep the fly in the strike zone.