Bowerman cut the waffle pattern from a spike plate, removed the spike plate from the Osakas, and glued the waffle material to the sole of the Osakas. 

You can see Bowerman's thought process reflected in the sole of the shoe.

These are so early that he needed more than one piece to cover the entire forefoot of the shoe - the spike plate.

The size was limited by the size of the waffle iron.