Arrange your model at a slight angle in a chair. Establish that they will be comfortable to sit in this pose for an hour or so (with breaks). Before you begin your drawing consider the angle of the central axis that runs through the seated figure. Notice any twists or bends.
Block in the basic shapes. Look carefully at which planes of the body are receding and which planes or lines are parallel to the edge of your picture plane. This will help you establish the bulk of the drawn figure in relation to the space around it.
Identify a measured unit that will help with the scale and proportions of the figure. Draw the model from different angles and positions. Remember to look and measure with each pose.
Identify the possibility of foreshortening and make written notes. Is there more than one line of movement? The torso may have a slight twist to it.
The following is the only drawing I managed in the life drawing class using basic shapes. Unfortunately, I am not allowed to post the model so I can only make do with a standing post instead of a sitting one. The model was supposed to slightly lean on the stool but she was actually standing quite straight with weight evenly distributed on both feet.
I was presently surprised how easy it was to get the measurements right when the figure was breaking down in blocks. I have overlooked the relationship between the body planes and the edge of the paper. I shall play more attention to that in the future.
The only foreshortening characteristic I can identify may be the left torso is bigger than the left as the upper body was slightly twist to the left.