Instructions
Stock up on potions, steaks and other healing items before going into battle. Flash bombs and a powerful weapon are also very helpful.
Walk around until you encounter a Rathalos or Rathian. Defeat the monster in battle by stunning it with flash bombs, then attacking.
Walk up to the monster's corpse when it is defeated, and stand by either the tail or the body.
Select the "Carve" option that appears. Using this ability, you may obtain Fire Wyvern Marrow. You may carve the monster's body up to three times before it disintegrates, and the tail can be carved only once.