How to prune, and what is considered the stem? It would help if I can see a photo.
Tiger Lilies require such little care, the only reason you might want to prune them would be to deadhead the spent blooms at the end of summer, but you actually don't need to if you don't want to -- they'll fall off in the fall and the main stems will dry up (you'll want to pull those out or cut them down either in the fall or next spring). There will be a main stem from which several lilies shoot out, each on their own "child" stems. You can "prune" your tiger lilies one of two ways: cut each individual dead flower off by cutting that flower's own stem, or wait till all the flowers on the main stalk are dead and cut the parent (main) stem almost all the way down.
This article provides some more information about growing Tiger Lilies: https://www.gardeningknowhow.com/ornamental/bulbs/lily/tiger-lily-care.htm