Start by loosely wrapping a piece of tape around the bottom of your rocks. You're just doing this so you know how much tape you'll need. Make sure to wrap it around the base of the rock as you want it to stand.
Remove the tape and stick it on to the edge of a table leaving about half of the tape unstuck. This is just to help ease the burden of cutting sticky tape. With scissors, start cutting out little triangles. This will be the bottom edge of your ghost rocks.
Keep going until the whole piece of tape looks like this. You don't have to be precise with your triangles, either. Remember, these are ghosts. They're not perfect.
This is important now. Stick your tape around the bottom of your rocks again, but make sure the triangles point up. That way you won't have a weird, straight line ghost sheet and you won't have wasted all your time cutting triangles out. True story.
Also make sure to really press and smooth out any wrinkles, bubbling, or lifting on the triangle portion of the tape. It's not as important getting the straight edge of the tape on smoothly, because you're simply not going to be spraying down there. You just want the triangle edges to be as sharp and noticeable as possible so by pressing and rubbing them down, you're reducing the chance of paint bleeding under the tape.
Place your rocks, one at a time, into your box so you can spray paint them. Make sure to work in a well ventilated area and wear a mask if you've got one.
After they're dry (which takes no time at all), remove the painter's tape. If there is some slight bleeding where you removed the tape, don't fret. I had some, but I thought it added to the wispy ghost nature and I didn't mind.
Now's the fun part! Grab your marker and draw on some faces! I chose to draw some really simplistic faces, but you can do all sorts of things. You're only really limited by your imagination.
I found that, depending on the rock, you might have to do two coats of marker on the faces just to get a nice dark line. Just wait a bit for the marker to dry and reapply again. Also if your rock is pock-marked (like most rocks), you can kinda dab the pen in those little holes just to make sure there are no weird white dots where you want black.