The only way to direct mount that humbucker without a ring is to make a small filler block or use something like the PMS system from FU-Tone.
The other route they offer is called "wood-mount" and doesnt have those recesses. If your body has that then you can directly mount them in to the wood.
As for the actual process, I use these screws from Allparts:
Mounting screws, under pickguard for neck pickup (6 pcs.), Stainless steel, 15/16".
www.allparts.com
They are a #3 screw which will have a tight fit into the humbucker mounting holes but will not damage the threads preventing them from being used in a ring or pickguard later. I also use the same screws for the single coils.
For height adjustment, I use foam weatherstripping that comes on a roll. Its 3/4" wide and 1/2" thick and I simply place a small section under the pickup.