I used polyurethane board for the tray ( I made it stronger by attaching metal bars beneath ,and painted him with white color because he was originally yellow )

Made some draining holes ( not a must ) especially under the place where the hill with the tree will be and another one for the opposite hill

Spread some crushed tuff and stick 3 wires to help support the hill

Then surrounded the tuff with Mud and added more tuff

In the 2nd picture you can see at the back the tree and the rootball

Placed the tree rootball on top of the crushed tuff

Covered the rootball with more Mud

I prepared the rocks to be used ( split them from bigger rocks using a chisel ) and if you want to be perfect you can use diamond saw to make the rock bottom flat

Positioned the rocks on the "Hill", in order for the scene to look realistic, it's important that most of the rocks volume will be inside the ground

I've created sort of a path from the bottom, between the rocks to the tree nebari, hopefully to lead the viewers eyes via this path

This is how it looks after this step

in the next steps I am planning :
1. add moss to this hill
2. create a second smaller hill on the right side
3. create an artificial river that will flow between those hills

I would LOVE to hear what you think, feel free to comment
Any Suggestions / Ideas will be welcomed