Quoted from KenH:

If you have some basic skills, all you need to do is
1. Wire a 5v relay coil to the existing connector(by the tilt mech).
2. Solder a wire to the hot side of a 50v coil and bring it down to one side of the knocker coil.
3. Run a wire from the other side of the knocker to the N.O. (normally open) relay contact.
4. Run other (common) relay contact to metal power box(by coin box) and terminate under one of the screws for ground.

Quoted from KenH:

In this case, the protection diode across the relay coil in not needed, because protection is already built in, in Stern's driver circuit. Although, it probably wouldn't do any harm to have one in there, as long as it's pointing towards the +5v (banded end toward the red wire), so I added one to the picture.
There is also, a protection diode across the Knocker, since it would protect the relay contacts.

Quoted from KenH:

I tend to agree with you, that the snubber should be included. But Stern's schematic in the manual showing a 5V coin meter (basically the same as the relay we're using to trigger the knocker) shows no snubber. I don't have 100% faith in that, as they could have inadvertently left the diode out on their schematic.
At any rate, it should be safe to add it in--even if it *might* not be needed.