Training Manuals are necessary items you will need in order to Evolve your characters in Fortnite. Unlike Pure Drops of Rain, which you can obtain as a common Mission reward, you will have to make sacrifices to earn Training Manuals. You see, the main way of obtaining Training Manuals is by retiring Rare Characters (Blue Status or Higher), Survivors, and Defenders. The higher the quality of character you are retiring, the more training manuals you will receive in return. You can even use Transform Keys to upgrade a Character's Rarity. However, once you retire a character, you will no longer be able to use them.
It's worth noting that you can also obtain Training Manuals by leveling up your Collection Book. However, since you only obtain them when you achieve specific levels for your Collection Book, the mentioned already listed is far more dependable and superior.