First Step - Find the right formula.
We were told to use
7 cups of River Sand

3 cups of Lime
and 1 cup of White Concrete

Once the formula was done and approved (after hours discussing if it was too grey or not) we started by pointing the rocks in the fireplace.
First wetting the rocks other it would dry up the water from the mixture.

And then start pointing

wait for it to dry a bit and brush the rock with a metallic brush, one by one. This is a VERY slow process...took us almost 2 days to finish just the fireplace .

Fireplace done, next week we will do the other Rock wall, and maybe with luck the lanbournes as well !