Here's a wee test/demonstration piece I have lying around to physically explain different techniques. It's a portion of an abandoned painting that has been used to explain glazing colour over monotonal underpainting, impasto, opaque into transparent velatura/glazes, dry brush and many other fiddlesome little things. I'm starting to become quite fond of it now.
Here's the basic underpainting of a moth hidden away in the big red Painting. It's tiny! Needs to be more grey and brown in colour. Possibly a slight shadow.
I'm then going to use a extremely lightly saturated glaze of viridian green around the whole painting to almost imperceptably deaden the red and lead the eye to the area around the moth.
Then a frame