>>2938770
Time to get started with the grout.
Any grout will do, unsanded is usually seen as mildly better than sanded due to a tendency to spread moisture better. If you have the choice between "flex" grout and normal grout, get the normal.
We'll get started with a very small amount of grout, as we only want to seat the tower for now.
For this step we'll need the grout, wet sand, a container to mix the grout in, cooking oil and the water tower.

First you'll want to oil the container with the cooking oil. This oil will be our mold release later, so make sure it's everywhere.
As for the grout consistency, it's your call. Thinner is easier to pour, thicker is easier to mold.
The general workflow is: Add wet sand to where you don't want grout (tunnels), add grout everywhere else.
Add some sand on top of the water tower, so that the water can evaporate from there and have a path to it.
Keep going until you reach the level of the water tower's cave roughly. It's a bit messy obviously.