It is easy.
Step 1: 1.What You Need
-2 different playing card packs (like solitaire cards and pokemon cards XD)
but these cards should be same sized
-and that gum looking sticky thing(or something else if you have a better idea)
Step 2: 2.Making Cards
Then put other card from your second pack on it(Make sure it is facing up)
You need to make about 10 of these cards
Step 3: Almost Done
You now should have that card pack bigger.
If it is bigger then take some normal cards from it and put them away.
Step 4: Lets Go and Make a Prank
1.Show your friend what card game you want to play and the cards(but dont show those 10 cards what you put on the top)
2.Say that you already shuffled that card pack.
3.Then start the game and each player gets 5 cards at the start(If you need 6 cards at the start then you need to make 12 of these cards)
4.Your friend may be little confused about those cards and may look other card in the deck.
5.You just say "April Fool" and the prank is done!
I hope you liked it :D and have a nice April Fools day