Do not try to drag this out too long. You might get to the point where someone in the group wants to either report what they saw to the ranger or head back for their own safety. Sound reassuring, and be sure the people you are playing this prank on can take a joke.
As soon as you emerge from the woods, begin laughing and let everyone in on the prank. Reassure everyone that it was just a Halloween prank, and then hopefully have a laugh about it. Be sure to pick up the bloody T-shirt before you move on. Because no other group playing behind you would have heard of the bloodthirsty murderer near the course, most people will ignore the blood on branches and grass. But a T-shirt might get someone's attention and could cause a panic.