First thing's first: Make sure you have the trick coin in the palm of one of your hands. In order to make a seamless swap you can't be reaching into your pocket.
Explain the purpose of the trick and ask for a quarter from a spectator.
Accept the borrowed quarter in the opposite hand of the one with the trick coin. This gives you a real coin in one hand and a fake coin in the other.
While speaking to the audience about what you plan to do, or anything really, you are going to pretend to drop the real quarter from one hand to the other.
In actuality, you are going to cup the real quarter in your hand when turning the hand over and making the fake coin pop up in the bottom, or accepting hand.
Though it took two steps to explain how to make the actual swap, it needs to be done quickly or else it will be evident something is amiss.
As you then show the actual quarter (trick coin, even though they think it's real) pocket the real coin.