I haven't looked at the sites you have, but i have spent 15 years as a vet nurse, given many pets intramusclur injections
and injected my skinny thighs for 6 months with avonex without too many problems
so i thought i would reply.
I think it is important to sit back on your chair pre jab.
Supporting your muscle within a soft grip with the non injecting hand is useful, ( and avoids any bone contact )
You need to go quite deep as it is a muscular injection, but if you don't feel the outline of the muscle, i would have thought this would
make the process harder.
I have vey skinny thighs ,even more so since being on a low fat diet, so i feel it is important to adjust to your body size.
If the site said 'pinch' the skin, i would say it is wrong, but a soft supporting hand on the muscle would be vital.