Position the things
Positioning is quite important while performing any magic trick in front of a crowd. Make sure all of the spectators are sitting in an opposite direction and then place all of the things on a table.
Trick the audience with words
Although you will be planning to outfox the audience with your actions, you have to play with the words in order to divert the attention of everyone to something else. In this case, tell the spectators that you will be making the coin disappear, so that they fix their eyes on it rather than the glass.
Place glass over coin
Put the coin on the table and ask the spectators whether they can see it. Now, place the glass above the coin and murmur some words, so that no one can hear them.
Place napkins on the glass
Put both the napkins on top of the glass in a way that the spectators can see nothing. Continue saying some words in your mouth in order to give an impression that you are reading a spell.
Remove the glass
Count till three and remove the glass. The coin will be present on the table, so tell the audience that the trick has failed this time and you will try again. While everyone will be fixed on the coin, you can move the glass under the table and hide it.
Place the napkins on top of the coin
Hold the napkins on top of the coin in a way that the spectators feel the glass is still there.
Slam your hand down
All of a sudden, slam your hand down with force and tell the audience that you have managed to make the glass disappear instead of the coin.