Douse a Pyromane, either by kiting it into water or with an Extinguisher Grenade, then damage it down to about half health and it will let you mount. From there attack nearby creatures to set them alight and use Flame Absorb to pull the fire in, which is what fills the bar. It throws you off periodically, so douse it again and get back on. On Scorched Earth there is no water near where it spawns, so carry it to some with a Wyvern.