Injecting insulin is not difficult, but it does take a bit of practice. There are three main sites where insulin can be injected: the stomach area except for a 2-inch circle around your navel, and the soft part of your waist, but not anywhere near your spine; the top and outer part of your thighs, but not your inner thighs or anywhere close to your knee; the outer back of your upper arm where there is a pocket of fatty tissue.