how do you stop blundering?

When you blundered, play self-sure and let your opponent think that is a tactical plan.

Some people don't use their time efficiently. That is, they rush moves without thinking, don't look at threats properly and don't look at opponent's plans. The long diagonal used to be a blind spot for me, that is, failing to see an opponent's bishop threatening a backward capture. Discovered checks and pinned pieces can be easily overlooked. I guess developing habits of scanning the board for threats against your queen, checks that interrupt exchange sequences and counting in your head to the end of a multi-piece exchange sequence. Sometime sacrifices, especially Queen sacrifices for a mate can be overlooked as in the mind it seems naturally counterintuitive for the opponent to give up a Queen. Back row mates can be tricky at times so leaving an escape square can be a good defensive strategy preventing a back row mate blunder. I guess, reducing or eliminating blunders takes a lot of practice by reinforcing good habits with routine observations adopted before every move. Looking at beginner's play, they usually are only thinking of their moves and don't consider the reason for the opponents moves. I still blunder but have reduced them gradually. Hopefully I will eliminate blunders eventually by following my own advice.

Don't practise take-backs.

I second both doing more tactics puzzles and not sending takeback requests. You'll be punished more by blundering when you refuse to send takebacks, so you'll learn to blunder less often. I also +1 focusing on classical time control instead of blitz. Not only is it good to learn to stop blundering, but a good way to play for beginners in general.

thanks to all of you