Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is my Jungle amumu guide and has never failed me.
When you get to the Thornmail and Force of nature you only buy one of these and it depends on what team you are playing against.
I always start at blue buff, I then move onto the wolfs then wreaths, then small golems then Red buff.
If I can I will go into their jungle to take there blue and the other creeps.
With this build by level 10 you can solo Dragon. At level 4 you should be able to easily gank and level 6 you're a killing machine.