Make sure you have a regular 52 count deck of cards, no jokers. Shuffle and have an observer cut the deck for authenticity. Next, place the deck face down. My picture just illustrates I have all cards

From the face down deck, roll over the first card. Whatever that card is you will place subsequent cards on top of it and count up until you reach the number 13 assuming jacks are 11 and queens are 12

For example if an 8 comes up first, then add a card (9), another (10), another (11), another (12) & another (13). Keep forming these piles of 13. If a king comes up first, shuffle it back in.

It may work out that you have just 2 cards left....say a 9 and a 4.....you do not have enough cards to count up to 13 so you will hold those 2 cards in your hand to complete the trick.

Make sure all stacks face down, making note where your leftovers are, mine are bottom left. However many stacks are left, ask someone to eliminate any number of random stacks so only 3 piles remain.

The piles that you discarded, stack those ontop one another and set them underneath the cards that couldn't add to the last 13.

Of the three stacks that were kept, flip over two cards and add the total of them together and add 10. In this example we have a five and an eight for 13 plus 10 to equal 23. Remember this number.

The leftover cards will be placed down counting up to the important number told to be remembered. Once you reach the number, count the remaining number of cards in the leftover pile.

However many are left in the leftover pile will always equal the face value of the third card not flipped over on the stacks. Done right, this works everytime.