Find a good location. Tennis courts with a large practice wall and school playgrounds are two good places to practice. If neither of these locations are available, you can also use a garage door or the side of a building, but make sure there are no windows or cars around and that you get permission to use these areas.
Choose the right ball to practice with. If you have beginner or intermediate skill levels, you should probably use a tennis ball to practice, but if you have advanced skill levels, a racquetball will give you more of a challenge. You can also use bouncier balls for a tougher challenge. A typical baseball is not a good choice, as it will thump off the wall and not provide much of a challenge for even beginner fielders.
Stand between 4 and 7 feet away from the wall. The closer you are to the wall. the faster the ball will come off and the quicker your reaction will need to be. Simply throw the ball off the wall, field it and throw it again. If you want to practice quick ground balls, throw the ball lower on the wall. For line drives, aim higher on the wall and throw the ball harder. For the best practice, throw the ball off the wall at different speeds while also varying the height and angle of the throw.