Pokémon Ranger/Catching Pokémon

First of all, tap the stylus onto the touch screen to activate the capture styler. Now, without lifting the stylus off the touch screen, draw loops around the target Pokémon. When you do so, a number appears above the Pokémon. This is how many loops you have to draw around it to capture it. When the number changes to an "OK", lift the stylus off the screen to capture the Pokémon. You can keep circling the Pokémon after the "OK" shows, which might net you some extra exp. You can also get more exp by circling more than one Pokémon.
 * Basics of Capturing

Most Pokémon won't just stand around while you circle them. All of them will wander about, and if a Pokémon wanders into the capture styler, it will break your line, making you have to start again from the beginning. Most will also attack, and if an attack hits the capture styler, then not only does it break the line, but it also does damage to the capture styler (one Pokémon in particular can damage the capture styler just by running into it). But so long as you are wary of them, then they are generally easy to avoid.
 * Pokémon Reactions