How do I start blundering less?

If you're talking about one-move blunders, where you forget that a piece is hanging, you're putting a piece on a square where it can get taken, or you miss a mate threat, you can self-analyze one of your games, pause for a moment, and try to recognize as many 'threats' as you can see, for both you and your opponent. Do this a few times and you'll get used to spotting these threats faster and therefore avoiding blunders.

When games get more complicated and the blunders you make follow from a sequence of moves, then tactics puzzles are a great exercise.

Play longer games than 10 minutes per side, and really use your time to check if your move is safe to play. Apart from that, you will blunder less as you play more and more games.

Play games long enough that you can practice doing this:

https://www.chess.com/forum/view/for-beginners/the-most-important-concept-for-all-beginners

Well, I didn't count them all, but it looks as if you played 20 or more games YESTERDAY. Instead of playing one game after another after another, set up a longer time control--say, 15/10--and then take some time after the game to replay it and really look at the moves. See where your opponent took advantage of your weakness. Look to see where you dropped a piece and think about what you could have done to avoid it.  20+ games in one day is no way to improve--unless, of course, you're just having fun fooling around and don't really want to improve.

Look for:

1. Checks
2. Captures
3. Threats
4. What your opponent is try to do
5. The move you want to do is safe

T a k e   y o u r   t i m e .  but you're about 800 rapid, maybe learn a opening.