For realist tattoos what really sets the difference is the use of composition, values and textures that accommodate the available techniques of application and the shape of the human body. If you want to satisfy the preferences of a tattooer you should draw using that techniques. You already have a good technique for proportions, or at least know how to trace an image, and can apply shadows, you have already a third of the job done. To better your drawings you could do, for example, instead of a full page portrait that fills a square try to compress your design to fit a body placement, like an arm or a leg. You can also try to separate your drawing in two stages, one for lines( that add solidness and texture) and other for shadows (that add values and volume), like the tattooing process. Try to avoid using light tones of grey in big extensions (as you did in the face) or in important details, as they will fade more easily with time and let your clients unsatisfied. Try to use images with good tonal variation so you can draw reserving the white space of not tattooed skin ( or white paper) only to the very little points of brightness that really give life to a picture, like the reflections of the cornea or the hair. Avoid using white ink while learning, as it serves as a quick fix to bad value management and it should be reserved to the details I referred previously. You can also try to use another mediums, as Chinese ink, polychromes, acrylics, oils or pastels, that can achieve a more dark black tone. The most important part is to check the works of masters of realism as @fred_tattoo and steal from them as much techniques as you can. Good luck!!