If either side has any Deep Strike troops in reserve, then when they become available, they may deploy anywhere on the table. This can be by teleporting, flying troops swooping down from the skies, creatures being summoned from the warp, tunnellers emerging from underground, or any other means of extraordinary movement.
Roll a Scatter dice. On a 'hit' the marker is on target, on an arrow the marker is off target - move it 2D6" in the direction shown. Once the final position of the marker has been established, place all the Deep Striking models on top of it, starting from a single model covering the center hole, then fully filling out concentric circles until all models are placed, facing in any direction. If a model can not be placed due to terrain (or any kind other than open), or another model etc, it is counted as a casualty.
If you roll a double 1 the squad has been lost - victims of a terrible accident, lost in the warp, shot down by enemy fire or crushed underground.
If the marker misses the table, then the Deep Strike troops are not available for the rest of the battle.