This are the tools I'm using to carve the rock
the start is by creating a big hole that will be used to place the tree roots

The 5kg hammer is just for a measure :)

Use the 5Kg hammer to shape the rock into interesting shape

Use a small hammer and a chisel to carve high details in the rock

It helps if you have a creative child :)

Support the rock in the right place and position in your tray

Use cement around the base of the rock

Let the cement dry and then you can release the support

Use the fractions and dust from the rock to hide the cement

Carefully position the tree in the hole you created before

Add small rocks in varying size and distance between each other

Add moss on different places to make everything look more natural

It is ready :)
I hope you've enjoyed and it was valuable for you!