To cut the backsplash tile on a diagonal, use a speed square and cut from corner point to corner point. Cut only as much diagonal tile as needed. The backsplash tile for this particular project was 6"x6" — so after being cut on a diagonal, each piece measured 8-1/2" diagonal point to diagonal point. This means that Paul only needs about 1-1/2 diagonal pieces per foot of backsplash.
Once the majority of the backsplash tile has been cut to fit, apply the thin set.
When you come to the edge of the layout and need extra support to hold the edge tiles in place, use nails lightly tapped into the wall to temporarily hold the tiles in place while curing. Also, 1/8" spacers were used for the backsplash.
To install the window frame tile for a small area, back butter the tile with thin set since the trowel can't fit.