I actually saw a junkie do this once in a back alley using a cars side view mirror to see what he was doing. Counter to bow-vipers advice he pointed the needle upward, I am not suggesting this is safer, just the only way I have seen it performed, but it was by a homeless junky in a dodgy suburb of my city. Unfortunately I have no experience or factual advice to give you on the subject. Be careful bro! Don't want to end up like the scum bag I witnessed doing this (he actually robbed me for 20 bucks the night before on a weed deal and by chance I ran into him the next day, before pounding him he said he would hook me with some heroin which I thought was fair, he ended up trying to convince me to drinkthe leftover water in the bottom of his coke can, I didn't want to start shit with a junkie holding a used syringe and by the time he disposed of it he informed me that he had hepatitis, FUCK THAT I just left).
When I aked why he injected into his neck his only response was "because it goes straight to your head", although I suspect he was trying to come across as hardcore and it was really because he had no other veins left, he was atleast in his late 40's and obviously a hardcore old school junky, that shit is real silly mate and if you have used up every other option maybe its time to stop using yourself as a pincushion, not passing judgement, just some sensible advice that is obviously easier said than done in your position.