How do I stop blundering?





Play a slower time control like 15-10 and make it a habit to look for every check and capture on the board, including the ones that would pop up as a result of the move you are planning to make. Even checks/captures that are initially bad can matter later or have tactics behind them.

I haven't even heard of these Chess Fundamental videos... That is helpful! I'm making headway through the Lessons here on chess.com, and it does help me recognize a lot more when my 9yo beats me. But after developing some pieces and castling, I wonder what to do next? I mostly just react. But it's still fun to continue learning and improving.
When evaluating, try not to focus all your time in only one idea. Tunnel vision is the main reason for blunders in my experience. Solid gameplay wins more games than advanced beautiful moves.

But after developing some pieces and castling, I wonder what to do next? I mostly just react.
At your level, you push wood.
Your pieces control certain squares of the board, and your opponent controls certain squares on the board. Literally make any move that lets you control more squares, preferably in the center, without hanging any of your pieces.
The idea is just that chess is a draw in perfect play. The only way to win is to take advantage of your opponent's mistakes. The more squares your control, the more chances you have for your opponent to place their piece in one of those squares that allows you to take it.
Once you are ahead, try to make even or better trades until you get a chance to queen a pawn or your opponent doesn't have enough pieces to prevent a checkmate.