One method is to stand at the top of rucksack if it's laying down with straps up. Put arms through straps, and bear hug it. Then muscle it over your head and the right way up.

Other is to tighten you dominant side should strap, and loosen then other. Stand feet apart with fore leg bent so can pick up the ruck by the straps and sit in on your thigh, then get the dominant arm through the tight shoulder strap. Should be able to fish for the other lose should strap and get it on.