mrsjtwalker posted two great threads on newborns using a back carry.
Here is the link to her picture with her 1 week old daughter:
Newborn SHBC pics. As well here is a link to the video she added of a
Newborn in a back carry.
I have never attempted to wear a newborn on the back, but I might just have to try when the next one gets here

For me the easiest way to do a back carry is to lay the wrap out on the sofa and then place baby right in the middle of the wrap. Then I would sit down on the edge of the sofa with babies bum touching my lower back. Then I could grab the wrap and yell for totingtots to help because I was stuck LOL
No seriously I did eventually get it, but it took a bit of practice.
She on the other hand would just swing the baby up on her back and wrap him right on.
Here is a good video of what looks like 5+ month old being wrapped.
wrap a baby on the side and the back I think the key is just practicing a bunch.