They are made the same size as a standard post card, 4" X 6".
If you keep the total thickness of them under 1/4" they can be sent through the mail, first class. Mail in an envelope or mail as you would a regular post card.
If you make them thicker, then there is an additional fee and they must go in an envelope.
You can use batting if you like.
You can applique on a plain piece of fabric or over the top of piece work.
You can make it in a crazy quilt style.
There is no right or wrong way to make these. The only limit is your imagination :D:D:D
Google: fabric postcard images
You will see hundreds of pictures of them :D:D:D