Each player has 21 shapes. Your first piece must cover your starting corner. After that, every piece you place must touch one of your own pieces corner-to-corner, and may never sit edge-to-edge with your own color. Different colors can touch freely; pieces never overlap.
Pick a piece, drag it or tap the board to position, rotate/flip and nudge to fit, then tap Place. On a keyboard: arrow keys move, Ctrl+← / Ctrl+→ (or Q / E) rotate, and Enter places. If you can't move, you're passed. When everyone's stuck, whoever has the fewest squares left wins — with a bonus for placing all 21 (and a little extra if your last piece was the single square).