First nst at 34w 5 days

My dr wants me to start having weekly nst due to being of "advanced maternal age" until it's time for c-section at 39 weeks (had two previous c-sections). I'm 36. Just curious what I should expect, they said it will be done in their office & should take 20-30 minutes. Does anyone have any advice on how to make sure baby is moving, baby is usually sleeping when we get there as we live 1hr 20 minutes away. The car ride almost always puts him to sleep. Any special snacks that won't raise his heart rate to much but enough to make sure he's awake. I'm not on any special diet so any advice is very appreciated!