To get to the Cowbell Skull location in Halo Infinite, start at the Foundation level. Foundation is the second level in the campaign mode.
Continue to fight and complete the mission, and you will finally take “The Weapon”. Once you put her in your helmet you will take a light-elevator ride up and across. You will find yourself in a room. Walk through this room, passed the child holograms and into the next area.
Here you will come to large corridor with a few grunts and jackals. Do yourself a favor and kill those enemies first, because they can mess with you later.
Use your grappleshot on the 2nd of the pillars on your side of the room to get to the first enclave. Then look back at the 1st pillar you passed when you entered the room, and grapple onto the higher ledge. Finally, look up and towards the center of the corridor above you. You should see a structure attached to the roof. Use your grappleshot to carefully snag the structure, launching you up and onto the small opening.
You may have to try this many times before you get the angles just right. It took me about 15-20 tries until I got it right.
Here is a short 1 minute video of how to do it properly:
