This is quite simple, and will probably happen normally in the course of gameplay unless you are a God at this game. You're first directed to pick the blue fruit early on after escaping the castle, and you'll notice that they will then be scattered around the world roughly one flower per two "screens". You can carry five at a time, and they regenerate if you leave the screen and come back. You feed them by opening the Majin command menu (RT) and pressing B.
You can *only* feed them to the Majin when he is injured, and each one restores him to full health, so it's not something you can spam. However, if you've completed the game and still haven't unlocked this, simply run back and forth between a calm area and one with weak enemies and a flower. The south section of Tagalo Forest Checkpoint is a good place for this, containing weak enemies and a fruit in close proximity to the exit to another area (The Waterfall Powerplant, to the south). Run in, let the Majin get beat up a bit, heal him (do this before you end the fight, otherwise the health he gets from fallen enemies may heal him before you get the chance to), grab fruit, rinse, repeat.