Setting the Screen
The player who sets the screen must position himself at least one step away from an opponent if that opponent is stationary and does not see the screener. The NBA and NCAA say the one step should be a "normal" step, thus leaving up to the officials' judgment whether the screener violated this rule. Essentially, the screener must give the stationary opponent a legitimate chance to avoid contact.
Leaving Enough Room
If the opponent is moving, rather than stationary, the screener still must be careful to avoid setting an illegal screen. The screener must give the moving opponent room to change direction or stop. The interpretation of this rule often comes down to a judgment call by the officials, since the opposing player's speed determines how close the screener may legally position himself. The NBA and NCAA say one to two steps away is typically enough to make the screen legal.
The player who sets the screen must not make illegal contact with the opponent. Primary examples of illegal contact include sticking out an arm, hip or elbow to hit or impede the opponent, or attempting to kick, knee or trip the opponent. If the screener remains stationary and square, and the opponent runs into his chest or torso, the screen is legal.
Once the screener establishes position, he cannot move laterally or toward the opposing player whom he is trying to screen. The officials will call the player for a moving screen and charge him with a foul. The screener is allowed to move in the same direction as the opponent. If the opponent is sliding to his right while guarding the player with the ball, for example, the screener can move along a parallel path and re-establish his screening position.