Those pictures are great. Yes I do the 'tent' thing too. You feel the needle go through the skin and then move freely in the pocket, as kanisha says. We find the best spot for us to be behind the shoulder, a few inches to the right of the spine and behind the shoulder blade. It's harder to immobilise them sufficiently for the neck, and they can too easily shake free as dyedinthewool says. We do them in our treatment race. She's other sheep in front of her and BH behind, my knee in front of her hip - she can't move very much. Left hand parts wool and tents the skin, right hand in with needle and push the plunger.
I've been stabbed by a colleague (abetted by a Swaledale) but haven't so far ever jabbed myself. Hence I know that Heptavac-P stings! (But only briefly.)
You'll get injecting guns from your local agri / animal health, or the vet. I've used quite a few different ones and all have been fine, but I prefer the ones where you have the bottle of vaccine hanging around your neck, feeding into the gun down a tube, to the ones where you insert the bottle of vaccine directly into the gun. I found the latter didn't refill correctly because the bottle was rarely vertical. So you end up underdosing and injecting air - not good.