We sleep-trained our first starting at about 6 months old, and we're about to do the same with our second. The strategy we used was some kind of incremental back-off. Put them down, light out, leave. Wait 5 mins. Go on, give them a little hug or back pat, leave. Wait 10. Then 15. Then 20. Then 30. Then stay at 30 until they fall asleep.

Next night, 10, 15, 20, 30, 30, 30...

next night, 15, 20, 30, 30, 30....

At some point it went up to like 30 mins for first check, then an hour for subsequent checks. I think if we'd gotten to that point we'd consider trying something else cause that's a lot of crying.

But in practice we never had to really adhere to most of the structure because iirc it was like:

Night 1: 5, 10, 15, asleep. Night 2: 10, 15, asleep. Night 3: asleep. Night 4: 15, asleep.

Then he was sleep trained and has slept like a rock with 0-30 seconds of fussing (usually 0) (and ~never crying) since.