Instructions
Purchase as many PokeBalls as you can carry. If Great or Ultra Balls are available, purchase those as well. They're more expensive, but they also have a greater chance of success.
Wander the wilderness until you encounter a Pokemon. Encounter rates are highest in tall grass and lowest on roads, so walk in the overgrown grass to improve your odds of finding a Pokemon to capture.
Fight the wild Pokemon until his health is reduced to just 1/10 full and the health meter turns red. If you can't lower his health that much, reduce it until the meter turns yellow. The lower his health, the greater your chances of a successful capture. In the process of lowering his health, avoid using poison and burn attacks, as these will constantly drain health, potentially killing the Pokemon before you capture it.
Cast sleep or thunderwave if you have a Pokemon with the spells. These spells not only incapcitate the Pokemon so it can't attack you while you try to capture it, but they also improve your odds of success.
Throw balls at the Pokemon. Start by throwing simple PokeBalls, but if you don't have any luck with them, use more powerful balls such as Super and Ultras. Some high-ranking Pokemon are nearly impossible to capture with a plain PokeBall, so using a powerful ball will make the process go by a lot faster. Once you make a successful throw, the Pokemon will be yours.