The principle is straightforward; you need to be able to grow the Mycobacterium host as a lawn on the surface of an agar medium in a petri dish; and filter the enviromental samples through a 0.22 micron filter.
Plate out the Mycobacterium and keep the plate open in the laminar flow for the surface to dry, then add then drop the filtered aqueous samples onto the freshly plated agar surface (in a grid pattern) and incubate the plate. You can then purify the phage from an plaque that form.