It's always important to make sketches. No matter how complicated the picture was, make a quick sketch, even if it may not be exactly like the original object (model), but you will see, again, quickly and precisely the difficulties that you are about to fulfill. It also happens that the sketch has come out quite successful and it can be used in the work (albeit with some correction).
I found a couple of such (sketches) that I used to work in 2015. I drew quickly, trying to capture the basic details and tone. In general, sketches correctly do based on the sensation rather than on the accuracy of the object, this will lend your work a liveliness, draw you with the help of traditional tools or in digital art.
If you have any questions, as always write in the comments, I'll try to answer!