The next thing is to do a drawing (more on doin' the drawing later) of our subject - I prefer to do this in charcoal on a separate piece of parer (i.e. not on my canvas) and transfer it to the canvas by rubbing the back of it with charcoal and "tracing" over it (you can have a second sheet of paper blackened with charcoal to male a "carbon" paper so you don't have to so this every time).
I do this because, if the painting goes wrong, I can start again without having to redo the drawing - I just re-transfer the drawing to a new panel!. Something else to remember is that it a total waste of time to put details in the drawing.
I then spray the faint, transferred charcoal drawing on my canvas with fixative - this has an advantage - if the (oil, not acrylic - acrylic when dry can't be removed!) painting goes wrong you can wipe if off and you've still got the drawing - and a disadvantage - the paint doesn't "eat" (absorb) the lines of the drawing - and they can show through - bad news.
If I have time, I prefer to go over the lines with thinned paint (burnt sienna/TOR) - transform them into painted lines
And now the absolutely crucial stage - the block-in or underpainting or initial layin or whatever you want to call it - 4 possibilities (= for a final painting that will be either acrylic over acrylic or oil over acrylic or oil over oil)
1) Monochrome block-in in acrylic or oil (the reason for using acrylic is that it dries, and when it does it's "iron" - won't be disturbed by painting over it (the reason for using monochrome is that you can concentrate on the values and work fast with just one colour)
2) A multi-colour block-in in oil or acrylic (the reason for this is that this color under the next layer tends to give a much more interesting final paint surface and you can do things like use complementaries e.g. a pinkish/reddish colour under skies or vegetation to a "vibration"/interesting effects. I'm interested in using a "watercolour" style technique for this block-in (Richard Schmid often seems to work like that). I love watercolour (more on that later, too)