I started off by making a pattern I drew by hand onto some craft paper.
I inserted some cardboard behind my sweater to make it flatter and also to protect it from paint bleeding through.
Then I cut out the pattern and trimmed it to fit on the sweater.
I decided to cut around some of the pencil on the pattern so that I would be able to trace the image onto the sweater better. This can be a bit fussy. I didn't do it everywhere…just enough to trace some of the image details onto the sweater. Then I pinned the cut pattern onto the sweater and traced the image with a paint pen. I just sort of slid my paint pen inbetween the slits I had cut in the pattern.
I went back in and filled in some areas and added a bit more detail free hand.
The one thing about painting on a sweater is that because it is a knit it was a bit harder to control the edges of the image. I would recommend just going slowly and using a paint pen for the finer details rather than a brush. I found it easier to work with.