To restrain her, sit her on her rump, leaning back against your legs. Draw the chosen back leg up and in so it's bent and tucked in to the body - this relaxes the muscle which makes it easier to put the needle in. Wait until she has relaxed a bit then separate the fleece so you can see skin. Aim for that bit. Tap the skin a couple of times then quickly stab in the needle. Choose the part with the thickest muscle so you don't hit bone. The bigger the bore of the needle (for example white not green) , the less it will hurt when you inject the liquid - seems counter-intuitive, but it's about the speed with which the liquid goes into the muscle - too fast through a narrow bore needle and it causes localised tissue damage. Your ewe's skin will be tougher than a lamb's so inject firmly, don't be hesitant. Draw back the plunger a little to make sure you've not hit a blood vessel. After withdrawing the needle, I gently pinch the skin together to close the hole so the liquid doesn't come back out. I don't massage the skin roughly, just a couple of strokes.
For choosing the precise spot, there's little help, but obviously you don't want to hit a blood vessel or more importantly a nerve. Think to a leg of lamb waiting to go into the oven and work out where, in relation to a living sheep, the major muscle masses are and where the vessels are, deep in the middle. In fact you are unlikely to hit a nerve in a well grown sheep, so just go for it. If you hit a bone, draw the needle back a bit as injecting up against a bone would be painful.