Never mind HERMS- he was just being thorough in his answer. For what we're talking about, here's the basic process:
You want to attach a ball valve to your 15 gallon kettle/keggle/metal vessel.
-order yourself a bulkhead from Pickles (PM him on HBT). He will include for $12 (plus small shipping) the bulkhead components and o-rings. Awesome deal- they're like $95 on McMaster-Carr.
-in the meantime, get the step bit mentioned previously from harbor freight or bargain fittings.
-when the bulkhead arrives, drill a small pilot hole in the side of the kettle/keggle where you want the penetration to be centered. then, use pilot hole to guide the step bit and create the necessary penetration to just barely allow the bulkhead to pass through.
-insert the bulkhead and screw it down tight so it clamps down on the keg.
-then, you simply just get the proper size and length nipple to connect the bulkhead to the ball valve.