Line up along the line of scrimmage in whatever position is required in your current offensive set.
Make sure that you're either "on" the line of scrimmage or "off," depending on what the play requires and what your football coach has instructed. Remember, it's a penalty to have too many, or too few, men on the line of scrimmage.
Determine your blocking assignment--the player on the outside of the defense who is most likely to get outside and make a tackle. Whichever player on the defense you're supposed to block will become the target of your crack block. The most likely target on any given play is the outside linebacker on your side.
Look as if you're going to run a pass route, right up until the play begins.
Watch for the snap of the ball, ready to go as soon as you detect movement.
Cut inwards as soon as the ball is snapped, as if you were running a really short slant route, and head toward your intended blocking target.
Get up as much speed as you can while running at the target of your block. Most of the players that you'll crack block will weigh considerably more than you, so you'll have to use your speed and momentum to really deliver a good crack.
Block the player you've been targeting with as much force as possible, and make sure that they don't get outside containment on your football team's play.