Once I have an idea and I have collected the reference material that I need then I often sketch a small pencil outline. I have a separate sketch book for this and sometimes I take this outline further measuring it out so I can scale it up easily and blocking in the light and shade. This is particularly helpful if I am combining several images. You can see one of my more detailed sketches for a picture of pandas which involved combining at least five main images and inventing quite a lot inbetween times!
I use acrylics for most of my paintings but I also sometimes paint in oils, gouache and watercolour. The colours I use are as few as possible which means they have to be based around the primary colours but still be able to create somrthing that approximates to black. Mostly I use prussian blue (or something similar), yellow (anything strong and bright without having any hint of orange), bright red and a white. Almost without exception the paintings you see on this website are painted from three colours and white.
No matter how carefully I plan in advance I do sometimes change things as I go along (did you spot that the line of the cow's back changed as well as the gate?). Sometimes I discover that I fudged something in the planning and I have to go back and figure it out. Then there is an important stage towards the end where the whole picture gets balanced up in terms of colour and shape. By that stage I am sometimes struggling to actually see the picture and it is best to leave it for a few days and come back to it. It can also help to take a photo of the painting and look at that, after hours of painting it is much easier to see the whole picture in a photo than in real life!
I'm almost never satisfied when I have finished. Perhaps that is just as well, the only time I was really satisfied with a painting it put me off painting for a few years because I didn't feel I could ever paint as well again! The painting from the panda prep drawing you saw on this page turned out alright but I didn't succeed in making it as powerful as I had hoped, I felt I could have done much better... and even though this makes me a little sad that's fine because, all being well, next time I will do better.