Your device will vibrate when a wild Pokémon appears in your map. To catch it, tap on the Pokémon to start a battle.
You will enter battle mode and the Pokémon will pop up in front of you.
Tap and hold on the Poké Ball
A target ring will appear around the Pokémon. The color of the ring determines the difficulty of catching the Pokémon. Red is extreme difficulty, yellow is intermediate and green is easy.
The way you treat the ball will determine it's trajectory. If choose to spin your ball before throwing your ball will arch left
You can also decide not to spin the ball and it'll only require a normal straight throw.
You'll have the greatest chance to catch the Pokémon when the colored ring is in it's smallest form.
Hopefully the Pokémon will be captured by your throw, but if not you'll have to prepapre to throw the ball again.