Below the picture of Mickey and the star bar is 3 pictures with numbers beside them. The first (gold mickey head) is how many lives you have. The second (picture varies with level) is the projectile and how many you have. The third is how many diamonds you have.
Collect projectiles scattered throughout the level and use circle or square to huck them the direction you are facing. You should achieve this naturally over the course of playing.
For those like me that to try hoard supplies, you lose all collected projectiles at the end of the second part of each set of levels; the third level is always the boss and you are not allowed any projectiles so use them up in the levels.
Any enemy defeated with projectiles contributes to the total of 75 enemies defeated for the Disillusioned trophy.