Measure the width of one shower, and mark the middle on the blue board with your pencil.
Draw a vertical line with your pencil and level from the bottom of the wall to the top.
Spread thinset mortar onto the blue board, over the bottom half of the wall, using a notched trowel.
Set the lowest course of tiles in place at the base of the wall, starting at the middle and working to the sides. Set spacers between them as you hang them. Also put spacers below them to separate them from the floor.
Cut the tiles at the ends of the row with a tile saw to fit.
Hang the next row up of tiles in the same manner, starting at the middle and working to the sides. Work your way up the wall row by row, cutting tiles around the shower fixtures as needed. Do the entire wall, and the other walls.
Allow the tiles to set overnight. Pull out the spacers.
Spread grout over the walls, from the top down, pressing the grout into the gaps and scraping it from the surface. Use a damp sponge to wipe up the excess grout. Let it set for 48 hours.
Caulk along the perimeter of the floor where it meets the walls, and caulk the vertical lines between the walls in the corners. Let the caulk set for 24 hours.