Fill in every grid square which doesn't contain a mirror with either a ghost, a vampire, or a zombie. The numbers round the grid edges show how many monsters must be visible along your line of sight if you look directly into the grid from that position, along a row or column. Zombies are always visible; ghosts are only visible when reflected in at least one mirror; vampires are only visible when not reflected in any mirror.
To place a monster, click in a square to select it, then type the monster's letter on the keyboard: G for a ghost, V for a vampire or Z for a zombie. To erase a monster, click to select a square and then press Backspace.
Right-click in a square and then type a letter to add or remove the monster as a pencil mark, indicating monsters that you think might go in that square.
Left-click on a clue to mark it as done (grey it out). To unmark a clue as done, left-click on it again.