OR mix by using your hands to rub the butter into the dry ingredients. If you grab a handful of the dough you should be able to squeeze it in your hand into one piece.
Add to pan & press down evenly. Use a rolling pin or glass to roll the dough to ensure it's tightly in the pan. Using a butter knife score the shortbread, basically marking the shortbread - you are not cutting all the way through,
Prick all over with a fork. If your fork sticks and pulls some of the shortbread up then gently use your clean finger next to the fork as you press down to kind of hold the shortbread in place.
Bake until lightly golden brown on edges. Sprinkle sugar on top and shake pan side to side to distribute evenly then while still warm cut on the previously scored marks.