i lay aston on the floor and put him in the harness, then pick him up (he looks a bit like a puppet...lol) then attach him to the spring...I find that is the easiest way, and safest
This is how we've got Harrison into his.

Harrison has just started in his today, at almost 5 months... Harrison has had great head control from birth, and he is constantly astounding the ECHN with his head control and alertness... but, it is only now that I have felt comfortable about putting him into the JJ... and he loves it!!!