Alright, here's what to do to get the PROPER scare factor from a nice and good jump scare.
Step 1. You need to have a very quiet environment. Keep it mostly ambient, like the quiet humming of building electronics or the buzzing of lights. Try and give as little forewarning as you can for the maximum effect.
Step 2. Occasionally, as the player walks, set up triggers to an ambient_generic which will play some sound in the distance. It'll probably scare the shit out of the player and keep them more and more tense for when the jumpscare is going to happen. I would recommend using the sound of a train whistle for maximum horror factor.
Step 3. Make a tunnel or hallway that the player has to walk through. Try to establish that the hallway is located deep within some building, in some underground tunnel, or anywhere, really, as long as it's somewhere you wouldn't expect a train to appear.
Step 4. Set up a trigger_once which will cause every wall at one point in the hallway to suddenly explode, and have an ambient_generic blast
this song at maximum volume to the point that it's nearly deafening. Then make a func_tracktrain which will blare past the player and blow up everything.
But in all seriousness, all that making jumpscares would take is some crafty enemy placement and some forethought on where your player is. And sudden loud noises will always make a player jump. Jumpscares are fun.