I did this when I first started doing a back carry.
I laid out the ergo on the bed and laid DD on the bed so that her butt was near the bottom of the carrier (not the waist strap). I sat on the edge of the bed and leaned back until I was almost on top of her. Then I quickly did the waist and arm straps and sat up.
I only did it in the house to get used to having her back there. After a couple of times I got more comfortable and did the hip scoot method.
http://www.youtube.com/watch?v=y0aPV6XSlkY