Create an instance of PlayerManager
Create an instance of CustomPlayer

use method addCustomPlayer(UUID, CustomPlayer) to add a player to a database. This is only necessary during a PlayerJoinEvent.

use method getCustomPlayer(UUID) to a custom player.

CustomPlayer methods:
getXp(int amount) gets the XP from the database
setXp(int amount) sets the XP in the database

Database:
Execute SQL:
CREATE TABLE `player_levels`.`players` ( `ID` INT NOT NULL AUTO_INCREMENT , `UUID` VARCHAR(36) NOT NULL , `XP` INT NOT NULL , PRIMARY KEY (`ID`), UNIQUE (`UUID`)) ENGINE = InnoDB;