Markers are great tools for field sketching. They lay down an even area of tone. With a small set of shades of gray, you can produce fast value studies. The markers will quickly bleed through thin paper so either put a blotter sheet behind the page you are working on or sketch on heavyweight paper.
There are different types of grays such as cool, warm, French, or toner. Choose one type and use them consistently throughout the drawing. I like the warm and French grays. You do not need every value that they sell in the store but get yourself a selection of values.
I like to start with lightest values and build up to darker values. Remember that you can never go backward, once the ink is down it is down. If you do not like it just laugh and go on to another sketch. You also can add whites back in with a jell pen or a sharpie paint marker.
Click on an image below to read the details for each step and see the full size drawing.