You create the object like you would any other object which implements something.

isCancceled() is just a way to make craftbukkit ignore the callback once its finished loading the chunk.

Check line 60 of this https://github.com/ShadowDog007/LonelySpawn/blob/master/src/com/forgenz/lonelyspawn/util/PlayerSpawnChecker.java

In my example it waits for a chunk to load before it 'spawns' a player. If the player logs off between clicking respawn and actually being teleported to the spawn then there is no point in having the callback run.