When will it be ok to put her in a back carry in my old school Beco (sewn in infant support? I'm thinking when she can hold her head up .. I do not trust my wrapping skills to wrap a back carry, with my others, I had a few bottoms pop out and the wrap ride up
Eta: I did not back wrap with a stretchy, but a woven Lana
She shouldn't sink down in the carrier. I would try tying it higher and tighter than you think you need. It will stretch enough for you to put her in there.
Most manufactures recommend 6 months for a back carry in a SSC - but you can always make your own judgement about your child'e readiness depending on trunk and head control.
Hope that helps!