Preparation:

Take a quarter and file one side of it down so it looks like someone has bitten a chunk of it off.

Method:

Approach a spectator and ask them to borrow a quarter.

You have already the gimmicked quarter between your thumb and forefinger of your right hand.

Take the quarter from the spectator and quietly move your finger and thumb and switch the coins.

Their coin is now slid to the back of your finger.

source:bestmagictrick.info