Draw a '3D' pyramid in perspective

To find the center of the base (so that the top of your pyramid will be in the right place), you simply draw a line between each pair of diagonal corners. Depending on where your pyramid base is in relation to the horizon line, this might look quite odd - one line may be quite a bit shorter than the other - but rest assured that where they cross is the center of the pyramid base. (Provided you've drawn them accurately!)

You might like to also see the tutorial on dividing a square or rectangle with crossed diagonals