Get a license from the owners of the Intellectual Property that you wish to make merchandise of.

If they are even interested, expect a sizable contract on what you’re allowed to make. Also have a sizable amount of cash to pay up front (depending upon how popular the fandom is, and which company you are talking to), and expect them to take a decent percentage of your profit.

You either aren’t large enough to matter, or you get C&D’ed.

Creat art that is not copyrighted but a reference people understand. Like a tshirt that says “I love you.” “I know”. Without a star wars logo