Don't use anything pre-printed or generated. Write them using your own words what you were listening to the radio, when you were listening, and how the reception was.Listening over several days means more chance of getting a QSL card. Use UTC for the times. Adding what type of radio and antenna also helps. Avoid SINPO, SIO and other codes in your reception report. My example using snail mail.
Dear Voice of Malgenia,
Thank you so much for providing your program to the public. I have been listening on 12345khz at 0600utc for the last several days. Reception was excellent on 12/12, however on 12/13 reception was fading in and out. On 12/14, there was interference coming from the Chicken Radio on 12340khz and I had to use the narrow filter to listen to your programming. Programs I heard included the news at the top of the hour about your foreign minister visiting ####, a program about stamps, your DXers report, the Malgenia soccer team losing in the World Cup, and many other features. I look forward to future programs and will gladly send in reception reports from time to time.
I listen to your programming on a Grundig 950 using 30 meters of longwire antenna. I would like to thank you again for your excellent programming and I would like to request a QSL card if possible. My address and email are listed below.
Sincerely,
GPSBlake
1234 Main Street
Nowhere, SC 29000
gpsblake@donteventrythisemail.cox
Some will say also include a self-addressed stamped envelope. And some stations will air your letter on the air (Radio Havana, Voice of Vietnam etc)