When drawing your tattoo design of a demon, it's best to think about what you want to draw, first you write or draw little sketches of what you want to have as a design, you could do this in a few ways with drawing little thumbnails or writing small notes and descriptions of what you want designed.
The main steps of drawing any tattoo, is to get inspired by the many available tattoos online and in any of your local tattoo shops. You need to find out what is popular and what would sell if you are interested in selling your artwork. The main point to consider is when selling your work, they are often bought in sets or what is called flash sheets of lots of collected tattoo designs. These can be on A4 size paper or larger A3 size and if you want to you can laminate them to make them look professional, this is especially handy if you are selling on Ebay.
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!
The next step is to flesh out your drawing development and see where you want the drawing to go, you yourself can push the design boundaries and create fantastic and other worldly creations. In this step you refine your demonic drawing so that it becomes more defined in shape, plus when you draw a tattoo you could design it in many ways. For example, drawing a long design that would ideally fit on anyones shoulder or a small design to fit on someones wrist, toggling between the different sizes and surface areas you can use to design your tattoos is a useful way of creating new ideas too.
Following on from this is adding details and other features that will help your design be either wanted by others or simple just appeal to others, you could add tribal or Celtic design elements to make your designs more professional. This should go back to the reason why I said look at other peoples work online and offline in the local tattoo shops, to see what others are doing, even asking your tattooist, like you should do - they will tell you and even advise you on what sells and what looks best in terms of tattoos.
Colors play a large part in the design of a tattoo, how they are shaded influence the buying decisions of customers online and offline who are searching for a tattoo. They are looking for something specific, so remember to make your demon tattoo stand out, make it striking to the eye, but not too colourful that your tattooist has trouble or some issues with it when transferring the tattoo from stencil to skin.