Yes of course Craig Mullins would have John Singer Sargent notes!
Straight from the notes:
Cultivate an ever-continuous power of observation. Wherever you are, be always ready to make slight notes of postures, groups and incidents. Store up in he mind without ceasing a continuous stream of observations from which to make selections later. Above all things get abroad, see the sunlight, and everything that is to be seen, the power of selection will follow. Be continually making mental notes, make them again and again, test what you remember by sketches until you have got them fixed. Do not be backward at using every device and making every experiment that ingenuity can devise, in order to attain that sense of completeness which nature so beautifully provides, always bearing in mind the limitations of the materials in which you work.
And some study advice?
Paint a hundred studies: keep any number of clean canvases ready, of all shapes and sizes so that you are never held back by the sudden need of one. You can’t do sketches enough. Sketch everything and keep your curiosity fresh.
Bonnie Ayotte recommended me using the plumb line as well:
When drawing from the model, never be without the plumb line in the left hand. Everyone has a bias, either to the right hand or the left of the vertical. The use of the plumb line rectifies this error and develops a keen appreciation of the vertical.
Thanks to Gothic Gerbil for typing it out and Jan Keirse for creating the PDF.