Instructions
Travel to any PokeMart, and talk to the PokeMart attendant to purchase mail. There is no difference between the various mails aside from the background image.
Select the mail from the "Items" option, and write your message. In the second generation of games, you use the D-pad and "A" button to select the letters you wish to enter into the letter. In the third, fourth and fifth generation of "Pokemon" games, you need to select certain words from a menu. Although this restricts the messages you can write, you are allowed to write longer messages.
Select the "Pokemon" option from the menu, and select the Pokemon to which you want to attach the script.
Select "Item," and select the mail you wrote. Your Pokemon will now be holding a script.