The hood is the easiest piece to start with, it’s a large, horizontal flat surface.
Sounds like you didn’t use enough slip solution on the headlight. You need to spray the surface with slip solution and the PPF as well. When you place the PPF on the surface, you should be able to slide the whole piece around like it’s floating. If you can’t, you need more slip solution.
With the headlight, get it positioned and apply tack solution to one corner and squeegee to get it tacked down. Then just work the rest of the PPF adding additional slip solution generously as needed to prevent the PPF from drying. If you get fingers add slip solution or tack solution and squeegee them out diagonally. You can also go back and squeegee out fingers afterwards as well. This is more helpful on big pieces like the bumper.
You really shouldn’t use a heat gun for PPF. It’s not like applying dry vinyl where you’re heating it up to stretch and fit. If the PPF was cut correctly it should fit with gentle even stretching. You should be using a lot of slip solution continuously with a mix of tack solution to tack as you fit the pieces. If anything use a steamer, not a heat gun.