The Grand Square action itself starts with the dancers who are far apart taking three steps toward each other, until they are close together. At the same time, the dancers who are close together take three steps backwards, until they are far apart. They time these steps according to the beat of the music, one step for each beat. After they have taken three steps, on the fourth beat of music, they all turn 1/4 inward, which again leaves them facing somebody.
Everybody does this three more times, each time with the dancers who start far apart walking forward and the ones who start close together walking backwards, except that at the end of the fourth repetition (i.e., on the 16th beat), they don't turn. This completes one half of an entire Grand Square.
They then reverse direction, following the same rules, going back along the path that they took originally, to end up back at their home positions. At the end, if the caller doesn't say otherwise, any dancers who are facing each other turn 1/4 to face the center of the square again.
By far the most common usage is "Sides Face, Grand Square". In this case, the sides will face each other at the beginning, and the sides will face back in at the end.