I do the same as Kim by the sound of it. You really need to see it, did you look at the video? I think it's on YouTube too.

I put the waist belt on and hold DS in front of me facing me, then put my arm over his head and scoop him around onto my back and lean forward a bit. Once he's on my back and stuck there by gravity I loop my arms into the straps.
It sounds and looks hard but just takes some practice.