In the cave when you first start, when you get told how to sneak, wait a few seconds for the goblin to walk over to fire and he will stand there until you kill him.

Then sneak around the corner and turn 30ish degrees to the right and put a elastic band around the left/right(depends which control layout you use) thumb stick and you should keep running into the cave wall.

You will have to keep checking as when you sneak skill gets to 25,50,75 and 100 it gives you a message and you stand still.

This is the way you can get your sneak to 100 before you even finish the tutorial.