INSTRUCTIONS:
Click on the President who belongs in the empty space.
Repeat until you have them all right.

hint:
It is possible to go in sequential order by clicking the first space to open it up. Once it's open, click the president who belongs there. Etc. This method is easier for some players, who use it a few times until they no longer need it. The game is harder (and a bit more gratifying) if you work out of sequence.