Dorothy, I heat set mine with my Iron. Just be careful not to scorch your fabric. (Which, I am guilty of doing. Note, I have tossed out the offending Iron.
) And, I keep my press cloth handy as well. I just keep the Iron over the written part for a few seconds.
Writing on the postcard is always the hardest part. I never know what to say. But, I always try to include a note on the back. It is strictly up to you, the maker of the card. Some people do write on it. Some people don't. I usually write out what I want to say on paper first, then I just copy onto my postcard.