I just need to know where their locations are
There exists a lot of videos on YouTube displaying the Skull locations in Halo 5. Here is a guide by Halo Completionist showing every single skull location.
https://youtu.be/g43PnvLLVyo
Note: I have noticed that it’s a bit easier to destroy the Pheaton on the mission Glassed with a Warthog instead of a Scorpion.