Buy the knee savers at a sporting goods store. If you are a catcher for a youth league team or a school team, you will usually be given catcher's gear to use. Your coach will have shin guards, a chest protector and a mask for your to wear. He may also have knee savers, but that is usually considered optional equipment and you will have to buy them yourself.
Place the pads on the back of your calf muscles. The shape of the knee saver is similar to that of a wedge of pie. The smaller end of the pie should be going toward the inside of your knee and the wider edge should be going toward the backstop when you are in a catcher's crouch behind the plate.
Hook the snaps coming from the knee saver to the shin guard. There will be either two or four straps coming from the knee saver. Those straps are meant to hook over the shin guard. Hook the two straps from the edge of the knee saver to the part of the shin guard closest to the knee.
Hook the bottom two snaps of the knee saver to the part of the shin guard that is closest to the middle of the shin guard. Connecting four straps from the knee saver to the shin guard will give your legs significant padding as you crouch in a catcher's stance throughout the majority of the game.
Unsnap your knee savers when your team's half-inning in the field is over. Take them off just as you would any other piece of catcher's equipment. Put them on before the start of the next inning. Putting on knee savers will probably take an extra 30 seconds per inning.