Instructions
Find the nearest PokeMart to you. PokeMarts are found in the main street of each city near the Pokemon Center.
Enter the PokeMart and talk to the left clerk at the sales desk by pressing "A". Tell the clerk that you wish to buy a letter by pressing "A" to the clerk's prompts. Pay the clerk 50 Pokemon money for the letter.
Press "A" to open the letter. A keyboard appears on the screen. Type the contents of your letter.
Click on the Pokemon you wish to give the letter to and select the "Give" option, then "Letter" option.
Find the nearest Pokemon Center. Go into the Pokemon Center and find the Pokemon Union upstairs.
Locate the person you wish to send mail to in the Pokemon Union and press "A" in front of the person. You'll be asked if you wish to trade Pokemon. Select "Yes."
Select the Pokemon with the letter that you wish to trade and verify the trade by pressing "A". Your friend receives the Pokemon with the attached mail message.