A guide assembly for use with a rotating bit when forming an opening in a stone countertop. The assembly includes a base unit having an annular, resiliently flexible body with a ring-shaped suction cup at its lower end. The suction cup forms a seal about the bottom of the base unit that forms a reservoir for holding water during the cutting operation. A rigid ring-shaped insert is mounted in the resilient body and includes a plurality of upwardly projecting mounting pegs. Flat, disk-shaped guide bushings are interchangeably mountable to the pegs atop the base unit, and have central guide openings of different sizes for different sizes of bits. The guide bushings are formed of a rigid, low-friction material, such as a low-friction polymeric material.