I think most of us got our start painting because of Bob's infectious Joy of Painting.  It was contagious.  If you were like me, I strove initially to do the painting in one sitting in less than 30 minutes.  We all eventually figured out that we just were not that skilled as Bob.  Once I figured that out and slowed down I improved dramatically.

So what is underpainting and how does it work using the wet on wet technique?

Simply put - Underpainting is a base layer of a "draft" of the painting sketched out directly onto the canvass.  I like to use a graphite pencil to lay out the basic shape of a mountain or cabin.  In traditional painting it is easier since there is not a base coat of liquid white and you can just draw a sketch on the canvass directly. That is ok no reason to panic, I generally put the very thin coat of liquid white on first and then I'll take either a lead or graphite pencil.  I'll trace out the basic mountain shape, or cabin etc right on to the canvass directly over the liquid white.  I find it especially useful with cabins to get the angles correct.  Should you get the shape, size or angles wrong- you just take the big brush and wipe the tracing away and start over ( happy accident ;-)).  I also found that when it comes to layering colors, shading, foreshadowing and highlighting I like using base underpainting to see if the colors work

Hope that helps!