Took me a while to respond but here you go. 

I can’t identify where my ideas come from as there isn’t just one source but they usually do come while I’m away from the easel, computer, or sketchbook. e.g. trying to fall asleep, taking a walk, a shower, or reading a book. A lot of times I’ll get several mediocre ideas and combine them into a decent idea or one idea will launch me into a better one that I hadn’t initially contemplated.  

Lately I’ve taken to sketching things out in photoshop as it’s much faster for iterating and manipulating compositions and references. I print the sketch (or prop up my ipad with it full screen) and then re-draw it onto gessoed wood panel. I then outline the sketch with ink or paint and build up several layers of paint washes on top of that. Sometimes I do the washes first and then sketch on top. After that i start adding in dark areas, texture, and building up the image. A lot of it is trial and error and everyone does it a bit differently. Develop a system, try variations, find a method that works well for you.

I’d recommend the book “Alla Prima” by Richard Schmid as a very good reference for painting theory and technique. Hope that helps a bit!