Once you have looked and seen what is actually out there, you can proceed with your tattoo designs from your initial idea or concept, whether you have written it down or drawn a small thumbnail sketch as a reference. I usually work this way and it provides a really useful point of concept design in stages and you could later see how your artwork has developed from the draft stage.
So to draw a demonic tattoo, you just start with the shape of your demon, drawing loosely you pencil in the structure of the demon, however you want it to look, you could at this stage just draw the head of the demon or the full image of the demon if you wish, it's really up to you!