Good day!
I’m not sure if there’s a guide for these particular part of quest for those who wants to be a Bullet Marker, but I’ll post it anyway. 
This guide is only for the part of getting Lightning Stone in Grynas Trails because this is the most time consuming part of the quest and a little bit of challenging. 
Below are the marked place where the stones will come out so you don’t have to keep on pressing the spacebar for this. 
(if there are other places where the stone will spawn, please feel free to add it here. thanks! 
spawning rate for each mark is atleast 10 mins and it’s random.
I hope that this guide will help you a lot in becoming a Bullet Marker.
Happy gaming!