Before Deployment, after choosing Deployment Zones, an army that includes units with the Ambush special rule must state which of your units with this special rule will use it (starting with the player that picked the Deployment Zone).
Deploy your army as usual, but without deploying any of the Ambushing units. Starting from Game Turn 2, roll a dice for each Ambushing unit at the start of each of your Remaining Moves subphases. After you have rolled for all Ambushing units, each unit that rolled 3+, now enters the Battlefield from any table edge. Place the arriving unit with all of its back rank touching the Board Edge. Ambushing models are free to move in the Remaining Moves subphase, except that they may not March Move, and they must end this Movement Phase no more than twice their Movement value from the Board Edge. If an Ambushing unit has not entered the board (due to failing all its 3+ rolls) before the game ends, the unit counts as destroyed. An Ambushing Character may choose to be deployed within an Ambushing unit that it would normally be allowed to join (declare this when declaring which units are Ambushing). In that case the player rolls once for the combined unit. Until arriving on the Battlefield, Ambushing units cannot do any actions at all, and all items, rules, abilities etc. do not work while not on the Battlefield.