This achievement is very easy to get with the NEW TUTORIAL WORLD
First, skip through the tutorial and make your way to the beacon. Open the chest and grab the 5 pieces of iron. Chop down some trees to make some planks. Make a pickaxe with the planks by turning them into sticks, then head to the docks. Grab two boats and head under the bridge and keep going straight until you see the desert village. Walk past the village and place your second boat in the water. Then take a slight right and keep going till you see a huge pirate ship. There will be a building directly behind it. Sail around the front of the ship and make your way to a set of ladders.
Find a room called "the library and hit the lever to the left of the fire place. Walk through the fire place into a room. You will find a solid diamond block. Mine it with your pickaxe, then use a crafting table to convert it into 9 diamonds. Your achievement will hopefully pop then!
Also if you need The "Diamonds to you" Achievement. Simply grab an extra controller, sign in as a guest. Then throw a diamond (
) to the guest!