Here's a tutorial on how to successfully
send a postcard! Hopefully it'll answer all your questions

First, in order to actually send a postcard, you'll obviously need to obtain one. Currently they can be acquired through
trade and presents.
Presents are currently found in
Vanilla rooms, where they will periodically spawn in maps, or you can also acquire them through
trade.
If you're confused on what the postcards look like in your
inventory, here's some images of them!:
(To access your inventory click Home > Inventory or click I)
In order to send over a postcard, you need to go into your inventory, click on one of them, then click "Use"
Psst. Make sure your mouse is
alive when you click on it or else the postcard
will not show up on your screen:
Once you click Use, it should bring up this screen where you're able to
click to draw on the postcard!
If confused of the buttons on the side, here are some descriptions of them:
Red X: Exits out of Postcard (Note: You keep it! You
don't lose the postcard when you do this)
Blue Paper: Clears out the entire drawing (Note: There's
no eraser, use this wisely)
Green Paper: Where you type in the Player's name (Note: Make sure your mouse is
alive! It
won't send if you're dead)
Once you click send, feel free to whisper the Player, if it's not a surprise, to make sure they got it!
Have fun with this consumable, I know I will