Hey there! Yeah, if you've never played Hearthstone, it might be a bit cryptic.
Basically: select a deck style (one of the paintings), discard the cards you don't want until you finish collecting 30 cards for your deck.
When you start the game:
0- You're on the bottom of the screen, your opponent on the top
1- There are Minion cards (the ones with 2 "tips") and Spell cards (the round ones)
2- Your mana is on the right side of your portrait. You get 1 point per turn, indicated by the small spheres.
3- You can click and drag cards towards the board to cast them. They will use the mana indicated by the top left corner. After you run out of mana, click "End Turn" on the right side of the screen.
4- Your minions can only attack on the turn after they're cast. To attack, click on the minion and drag towards the target. Minions marked as "Protector" (indicated by having a shield around them) need to be attacked before you can attack anything else. Minions can only attack once per turn.
5- To win, you must make the opponent's HP reach 0. To attack the opponent, target the portrait in the middle of the board, opposite to you.
Hopefully this and the UX helps! I'll probably put some extra work in accessibility during the week.