Sommary from wikipedia article:

Shortbread is a traditional Scottish  biscuit  usually  made
from  1  part  white sugar, 2 parts butter, and 3 to 4 parts
oat flour.  Other ingredients like ground rice or corn flour
are  sometimes  added  to alter the texture.  Modern recipes
also often deviate from the original by splitting the  sugar
into  equal  parts granulated and icing sugar and many add a
portion of salt.

Mix, roll out to the desired thickness, and cut  it  however
you like, with a knife or cookie cutter.  Bake in a moderate
oven until they just start to  brown,  usually  about  10-15
minutes, longer for thicker cookies.

It's OK to use wheat flour, though the texture will be a bit
different from oat flour.  Barley flour is also traditional.

Also traditional is the addition of assorted spices  (mostly
cinnamon)  and/or  nuts.   The  nuts  may be in large pieces
pressed into the top of each cookie, or may be  chopped  and
mixed  with  the dough.  Spices may be mixed in or sprinkled
across the top.  But many people prefer the  plain  version,
with its strong butter flavor.