You start by leaving a penny in the shoe of a person you live with. A wife or husband is a good choice.

Next day, leave a penny somewhere else, by the sink maybe. Or a nightstand, in the
middle of the room, just don't be obvious about it.

Every day, leave a SINGLE penny in a different place where your target is likely
to find it. SAY NOTHING ABOUT THIS.

He/she probably won't mention finding pennies until the 4th or 5th day. Keep leaving SINGLE PENNIES, ONE PER DAY, in odd places until your target mentions finding them, and isn't that odd?

DON'T BE INTERESTED. Act like you couldn't care less about this, and change the subject.

On the next day, start leaving 2 pennies, in different places. Be creative. ONLY ONE
LOCATION PER DAY. Now your target will start going crazy, trying to figure out what this means. He/she will probably look to the supernatural, thinking he's trying to be
contacted by the other side, etc.

If you want to be clever, use pennies with dates of special importance to your victim (birthdays, anniversaries, etc.) so much the better!

There is really no end to this prank, just go very very very slowly, and you'll have to come clean about it before your target calls the police or Sylvia Brown or some local witch doctor.

YOU ARE RESPONSIBLE, and you love this person, so you'll know when it's wise to
stop.