Cut a 2-inch-by-1-inch rectangle from both the red and the yellow cards. There is no official size requirement, but for practical reasons, the card must be small enough to fit in your pocket but large enough to be visible to all players when displayed.
Snip all corners of the red card, approximately 1/8 inch from the edge. Doing so enables you to tell which card is which while in your pocket. This is preferable because pulling out both cards when refereeing can cause confusion. Since the red card is less frequently used, it makes sense for this to have the modified shape.
Draw a table on the yellow card, with two columns and 10 rows. Use this column to note down the number of the player cautioned and the minute of the game in which you issued the caution. Noting down the penalties enables you to keep track of which player has been booked. At the 2006 World Cup, English referee Graham Poll made a high-profile blunder when he issued three yellow cards to Croatia's Josip Simunic before issuing a red card. Under the official laws of the game, the player should have been sent off once he was given a second yellow.
Lay a piece of self-adhesive film on a table, sticky-side up. Place the red card on top of the film, and score around the edge of the card, leaving a 1/16-inch margin. Fold the extra film over the edges of the card. Place the other side of the red card on the film, and score out a second, equal-sized piece to cover the other side of the card. Fold the edges over to fully cover the card. This protects the card from rain and gives it a wipe-clean surface in case it's dropped in the mud.
Cover the empty side of the yellow card. Naturally, you shouldn't put self-adhesive film on the side with the table, as you won't be able to write on it.