Keep the eraser away from any tape machines or tape that you don't want erased. Hold the tape you want to erase about 3' away from the eraser. Turn the eraser on. GRADUALLY move the tape to the eraser placing it on the platform. Rotate the tape on the platform for a few seconds and GRADUALLY move the tape away from the eraser. When it is about 3-5 feet away turn eraser off.
If it's like other Ampex 111's that I've seen in the past (long ago past), the tape sits on top of the eraser. My description is how you use one where the tape sits on TOP of the eraser, not IN the eraser. Here's a picture of one: http://www.bigdmc.com/ampex-bulk.htm