I did very gentle sleep training from day 1, when I say gentle I put him down whenever I possibly could but also accepted that it was cosy for him to sleep being held and that being held built our bond.
I initially used Harvey Karps 5S method then a mix of BW and Karp when I discovered BW at 4.5 weeks. DS was independent around 8-10 weeks. By independent I mean he was mostly put down awake and happily went to sleep on his own, he still needed help when he had reflux, teething, illness etc etc etc and any other tricky times.
The 'problem' I had was it was almost impossible to APOP, he could (but very very hard) sleep in a sling if I really forced him to, it was a nightmare if I needed him to sleep in a pram, utter nightmare. My DS had to be in his own cot on time for every nap or he had a melt down. With that in mind I would think about where you really want your LO to sleep, if you need flexibility (pram, car, sling, out and about) so that you can take DD1 places then begin that habit now. It's pointless having a LO who will only sleep in their cot if you really need them to sleep in a pram.