i think ergo says to wait until baby is sitting up independently. i would recommend waiting until your baby fits into the ergo without the insert (legs out) and has good upper body strength. in general it'll be somewhere around 4-6mo old, depending on your baby's size and development.
there are other carriers that allow for a safe back carry with a newborn or small infant, but ergo is not one of them. if you can't wait, look into a woven wrap, a podegi, or even a mei tai. as far as getting them up onto your back, it's all about practice! spend a few days trying different methods with a second person (spotter) to help out if needed. if you haven't already checked it out,
www.thebabywearer.com has forums like this that are filled with tutorials and great advice. i highly recommend visiting the boards and asking the mamas there any questions you have.
good luck and happy babywearing!
