More in Floors
For the stair risers, tape a piece of cardboard on the step to keep it free of thin set.
Sand the risers before applying the tile.
In order to get stone tile to stick to wood, be sure to use a thin set recommended for use on plywood for the stair risers if wood is the substrate.
Note: EGP thin set mortar is a thin set designed for use on "exterior grade plywood."
It's easier and cleaner in small spaces like this to back butter the tiles before laying them. There will be less mess on the stair treads by doing it this way as well. Be sure to wipe away any extra thin set with a margin trowel.