Fire a bullet from a human weapons nearly parallel to any surface that will leave a bullet hole (i.e. a wall or hard ground, not dirt or ice). At the point where the bullet hits, there will be a mysterious splotch of human blood. You can also use the method below.
Halo: Combat EvolvedEdit
Exit a level's boundaries by means of bumping (with a turret, for instance). Then, shoot any corner or edge of a wall with a human weapon, and the blood should appear. The method for Halo PC does not work on the Xbox version.