In act 5 scene 4 how does Malcolm's plan begin to complete the witches' prophesy?
Malcolm instructs his men to pull branches off the nearby trees and hold them in front of the men's bodies in order to conceal their numbers and also to make them a much more difficult target. As they march up the hill from the forest, they must have looked exactly like the trees themselves beginning to march up the hill to Dunsinane castle. This is the beginning of the prophecies. The remainder includes that Macbeth must beware Macduff (which he already does for killing Macduff's entire family, all his friends, and servants in his castle) and also that Macbeth can't be harmed by any man born of woman. Macbeth takes this to mean that he is invincible, but Macduff is not technically born of woman since he was born by c-section and did not pass through the birth canal.
One of the witches' prophesies was that Macbeth would not be defeated until Birnam Wood marched on Dunsinane Hill (Macbeth's castle). As Macbeth is in his castly frantically trying to defend it, Malcolm has his forces, located currently in Birnam Wood, cut down branches and tie the branches to themselves to use as camouflage as they advance toward the Dunsinane Hill. Thus, Birnam Wood is marching toward Dunsinane.