Just think of it as the first three lines of the story.

Type the Kicker (whatever you want in little text above your title). Hit Enter. Type the title. Hit Enter. Type the subtitle. Hit Enter.

NOW, go back to the title line, select some text, and hit the bit T till it won't get any bigger. Go to the kicker and hit the little T, and go to the subtitle and hit the little T. In the edit window, you should see in faint gray text: Kicker, Title, and Subtitle in front of each line. If you don't, just keep messing around with those big and little T things till it works.

It's not you - it's Medium. They could have designed that to work SO much more gracefully.