Mix a bucket of thinset to a peanut butter consistency (Image 1). Back butter the tiles with a 3/16" square-notch trowel (Image 2).
Set the tiles along the batten board using spacers to ensure even placement (Image 3).
To determine the size of the tiles around the top of the tub, set the tile on top and use a level to make sure it is flush with the front (Image 4). Scribe a line across the top of the tile so that the tile will fall just under the lip of the tub (Image 5).