TNR means "Trap, Neuter, Return".
A tipped left ear is a universal sign that a cat has been spayed/neutered and will never again reproduce.
Whether you're dealing with two cats or twenty, the goal is to get 100 percent of the colony spayed and neutered. Catching and fixing all the cats at the same time ("mass trapping") is the fastest way to accomplish this, but, you can move at a slower pace using similar techniques for one or more cats.
To read about the basics of TNR, read this PAGE from Alley Cat Allies.
To watch and learn the basics of TNR, watch this VIDEO from Alley Cat Allies.
To learn how to trap kittens and mother cats, read this PAGE from Alley Cat Allies.
How to Trap Kittens and Mother Cats