Once the first two rows are completed, we have to start aligning the bottom corners of the Cube, the 4 middle bottom cubes being fairly simple to align. Let me start with some definitions so that you may easily follow my subsequent arguments.

Fig. 19
An ideal corner is one whose side colours are identical to those of the little cube just above it. In the diagram, it's blue and yellow.

Fig. 20
A good corner is one which contains both colours of the little cube just above it but wrongly aligned. In the diagram, the corner does contain both blue and green.

Fig. 21

A special corner is one whose bottom colour is the same as that of the face to which it belongs. In the diagram, it's blue. By default, a special corner is also a good corner but it is different from the cubie in Fig. 20.

Any other corner will be labelled as a bad corner.