The mechanics of MetaSET are the same as for SET. Cards are laid out and everybody tries to find a metaset matching certain criteria. When a metaset has been spotted, the player announces this and then points to the cards that make up the metaset. They get a point if it is a metaset, and lose one if it is not.
What is a metaset?
For any pair of cards, there is a single third card that would complete a SET.
A metaset is any two pairs of cards for which the completing card is the same.
For example, consider the following layout:
For cards A and E, the completing card is a single purple half-filled rectangle.
For cards D and G, the completing card is also a single purple half-filled rectangle.
Therefore, cards A, E, D and G form a metaset.
The completing card itself need not be present.