This is a very popular portrait crop where the figure is cropped substantially below the waist and down to about mid-calf. It is an easier shape to fit to a canvas than an elongated whole standing figure and works especially well for women with long skirts.
Find a suitable reference or crop a whole figure reference to produce a three quarter length portrait. As you include more and more of the figure the head will be relatively smaller and the features will have to be painted rather differently; not so differently if you are working at a large scale but significantly less detailed at a smaller scale. The head is usually still the focal point of a portrait study so a good structure for the head and an indication of the main facial features are still essential.
The amount of detail with which the facial features are painted depends on several things;
The size at which the work is being painted, obviously the larger scale the more detail that is possible
The head should not usually be very much more detailed than the rest of the figure despite its role as the focal point. If the head is worked to a high degree of finish and the rest of the figure is not there is a risk that the painting will have no unity. In the worst case the head may appear so differently treated it may look as though it has come from a different painting
The over all look of your finished work; this is related to 2. If you wish your work to have a highly finished and detailed look, it should all be treated in that way. At the same time as in all paintings some areas should have more emphasis than others and there should be a focal point, in the case of portraiture usually the head. This may be achieved by more detail or greater tonal contrasts in certain areas but each area should relate to the rest of the painting to give the work unity. Look at how Modigliani achieves this and compare his works with those of Rembrandt.
Some portraits concentrate on the likeness and character of the sitter, while others may be more simplified or abstract. A glance at portraits by Rembrandt, Goya, Sargent, Matisse and Modigliani will give you an idea of just how varied the ways of depicting the clothed human figure are. Some examples can be found on the following Pinterest board: