how can i find or make a convincing card in envelope steal? in other words i ahve a stack of envelopes, spectator puts a card into the top envelope , ( card is secretly loaded into windowed envelope second from top instead) i hand them the top envelope in which apparently holds their card - hahah yay to me i now have their card

im sure that this is a classic move in the world of mentalism and are a bunch of ways of doing it.
io saw derren do this same type of load and steal in one of his live shows!
ive heard of something named the shaxon flap??
any direction would be greatful?
i have scissors and envelopes at hand now !!