Hey Burton,
I like touse almost twice as much reducer as usualt in my drop shadow black.
Like Rex said an airbrush is pretty much the only way to go. Just back mask whatever it is you want the shadow under and lay down the black.
Careful not to over do it. It's tough to judge sometimes because you can't see the piece your shadowing since it's masked off.
Overspray isn't much of a problem with the airbrush. I take fineline to mask off the edge I'm shading under, then use two inch tape and kind of bend it up so it makes sort of a wall to keep the black from blowing onto the part you don't want shaded.