With the caster bases locked in place by the braces, flip the table upside down. In this step, attach a caster to each end of the two (green) caster boards using 1-1/2 inch screws in the four holes of the caster's base. The caster should be centered on the caster base, and be positioned about one inch from the end of the board.
You do have some flexibility as to where along the caster board you'd like to affix the casters. However, in this case, wider is better. The greater the distance between the casters, the more stable the table will be when it is being moved.
Once the casters are all attached, return the table to it's upright position and verify that the table rolls around smoothly.