Honestly it was simply time. I let them decide when they were ready, and it just so happened to work out that for DD it was 6m and for DS 8m.
I tried putting them down in the crib and did a modified CIO. If they were seriously crying, screaming, red faced…I didn’t leave them there. I figured they weren’t ready, they needed more comforting. But most of the time, it was a fussy, whiny sort of cry, the kind that meant they were just tired. So I would let them “fuss” it out.