A net is a flat pattern of six squares that folds into a cube. Of all the ways to join six squares edge to edge, only eleven fold up properly; the rest leave a face doubled and another open.
Two quick checks rule most bad nets out:
- No 2 by 2 block. Four squares in a square can never fold round a cube.
- No five in a row, and no row of four with two squares on the same side of it, next to each other.
For opposite faces, use this: in any straight line of three squares, the two ends are opposite. Faces that share an edge in the net end up next to each other on the cube, never opposite. When there's no straight line to use, fold one square at a time in your head, starting from a face you call the bottom.