My advice:
- long flowy sentences as you build up the tension, then short choppy sentences once the action starts.
- focus on small unsettling details in your description
- how much does the reader know compared to the character? Nothing is more stressful than knowing more about a dangerous situation than the character does and not being able to warn them
- conversely, a lack of information is also terrifying. Withhold the sort of information that would make a character (and, by proxy, the reader) feel safe. Let them ask questions. Make them wait for the answers.
- is the character expecting to be in danger? Let them wait for it. Keep them on edge, maybe even have them feel safe before you strike.
- make sure the reader knows what's at stake, and make sure that the reader can't immediately expect everything to work out just fine
No worries! Happy writing!