Yeah instructions are non-existant for the moment :-)
You should select "Single Player" then choose a team and click "Start". You arrive then in the "match" view. Then you should drag and drop cards from your hand (at the bottom) onto the blue spots. Once you're done, press "End Turn" for the game to resolve attacks and make the AI play (on the red spots)
When you hover on a card of your hand, it shows you where you can play it. When you hover on a card on the board, it shows which spots it attacks.
Feel free to comment here, I have updates ready but I'm not updating the itch version at every dev iteration yet.