What is the best way to mount the projector?

Different rooms require different approaches. Side-Mount is preferred when the room allows it. End-Mount is the usual alternative. Vertical-Mount (lens pointing straight down) is for special applications only, and only with a projector the manufacturer approves for that orientation.

Side and End may be wall- or ceiling-mounted in a conventional projector orientation. Vertical is not the same as a conventional ceiling mount.

See mounting examples for photos and descriptions of several methods, and Requirements for height and distance guidance.