Fill a bucket with sand mix until it is 1/4 filled.
Pour water into the dry mix, mixing thoroughly with a trowel, until the mixture is still thick but easy to spread. Too much water will cause the mixture to become soupy, while too little means that it is hard to spread.
Wet the stone facade using a sponge.
Slather the mixture onto the stone facade using a trowel. Your first few passes fill in the gaps between the stones, while the successive passes create a smooth surface.
Make more of the mixture as necessary. Make small batches so it does not harden in the bucket while you work.
Allow the mixture to dry thoroughly. Depending on the conditions, this may take several hours or a full day.