No patching is used. The Minie Ball is hollow based. Depending on the type of mold, these cavities can vary in shape and depth. Most are cone shaped or rounded at the top. The original Minie used an iron cup or plug in the hollow which was driven forward by the explosive gas and expanded the thin skirt into the rifling. Pritchett, an English designer, used a boxwood plug to do the same thing. James Burton, an American, did away with plugs altogether after finding that the gases were sufficient by themselves to expand the skirt. There had been complaints that the plug tended to fall away from the bullet and presented a danger to friendly troops who may be in front of a firing line. (I'm speaking of troops such as skirmishers who would be on a lower level and not in the direct line of fire).
Patching was used on the Pritchett bullet. It didn't have grease grooves and the patch was actually part of the cartridge. It was left wrapped around the bullet
Another method of greasing Minies is filling the hollow base with lube. I myself don't recommend this method, however some people do. It will contaminate the powder, especially if left in the gun for very long. A friend of mine lubes his "trash can" Minies this way. This type, which is a wadcutter type, doesn't have grease grooves. In his case, the bullet is loaded and fired immediately. I must admit that the lube does get distributed in the bore quite well this way. The originals were not lubed in this manner.
