Instructions
Walk around Route 209 or Route 210, toward the south, in "Pokemon Diamond." At either location, Tauros usually has about a 20 percent chance of appearing.
Use the PokeRadar item. When a shiny patch on the ground appears, approach it---it may be a Tauros, or another type of Pokemon. You can get the PokeRadar after you see all the Pokemon in the Shinnoh regions and receive the National Pokedex item. You can still catch Tauros without one, but the Pokemon is harder to find without the PokeRadar's guidance.
Use the Pokemon of your choice to fight Tauros into submission. Eliminate as many HP from Tauros as you can without depleting its HP entirely.
Throw a PokeBall at Tauros by selecting it from the items menu. To increase the likelihood of success, try special varieties such as QuickBalls and UltraBalls. When you've successfully caught the Pokemon, the message "Gotcha! Tauros was caught!" appears.