First you need to aggro this fat guy. He needs to spit his brown bone throw up gunk and you need to get covered. He then will vacuum you up. Then you must drop loot when you are vacuumed. He then will go to sleep and you can ride him. You will go through a mini game where he requests certain things to eat. Keratin/raw meat/raw prime/wood/stone/metal. (I may be missing something). Do this three times. He kicks you off. You do the process again until tamed. I did this with flak armor on. He melts it pretty quick tho with other attacks. If you die after being kicked off you do not lose taming effectiveness and can come back to finish.

Sincerely, a guy who died 12 times doing this day of release.