The effect of this card trick
A really unusual trick, it has quite a lot of preparation but very little performance. The performer makes a claim to a number of spectators that they can actually step through a playing card. They then do exactly that.

Before you begin the trick
Use an ordinary pack of playing cards plus an extra card that you don’t mind getting damaged. You also need a pair of scissors. 

Take the extra card and ‘prepare’ it by cutting a slit in it lengthwise along the centre line. Make sure to leave a remaining thickness at either end of the card of 3 to 4 mm (just over 1/8th inch).
Fold the card along the cut you’ve made and then cut several straight cuts into it (still folded) – alternate the cuts you make by doing one from the centre slit almost to the edge and then from the edges almost to the slit. Open the card out and flatten it back into its original shape.
Hide this card in the pack. (It makes a very obvious gap and is easy to feel and find)