After getting the sketches and reference together I draw the lower legs, torsos and heads separately.
I draw the parts on tracing vellum, over my reference or rough sketches.
I then scan in the pieces and in PS clean up the work. Adjust the levels to get a nice contrasty image, then I select the white and delete it making the background transparent. Using a large red layer stroke I find all the crap that is partially a result of having a dusty scanner. Selecting the background again and then increasing the selection by 1 pxl usually removes most of the crap.