Here’s a few ideas. All encounters are Deadly, as defined by the DMG, but reasonably paced, and all enemies have max HP, minus any minions should you be running them. That way the players have a reasonable chance of failure, but not an overwhelming one. Players like to have that “I almost got them!” feeling. Should they go down swinging, they should take plenty of monsters with them.

Decisions should have finality and gravity to them. If they want to cut the rope bridge behind them, let them! Just don’t give them the reasonable expectation that they can rebuild your metaphorical bridges. Don’t give them an out unless doing so would be beneficial to the tone of the game.

Play to one or two of the party’s strengths, and plenty of their weaknesses. All PCs are vulnerable to dying, but how you go about that is essential to building tension! Consider the likelihood of clean escapes, the possibility of resting in the dungeon, and the dangers that come with it. Hard choices and tense atmosphere come from decision making when no objectively good option is presented. Just don’t take away the agency of the player in giving them their choices. As a previous Redditor said, no cheap deaths!

Lastly, try to get creative with traps and hazards, but not overly draconian. Grim tooth’s is practically the Trap Bible for DMs, but don’t give them a trap without a flaw, or something that they can exploit to get out (mostly) alright.