If your dolls haven't written to Santa yet, you should tell them to! Which is why I have a printable for you!! I will now tell you how to how to make an envelope!! Here is the link to the envelope template I found.
It took me a while to find the right size, but you will get it. Or make your own!! I just glued the corner flaps to the bottom and done! Use a small square sticker for a stamp, and write the address! Here is the link to the Santa letter image.