DS self settles at 18w which is 2 weeks ago I used to feed him, put him on his side and pat his back, rock him etc... until he's sleepy (but not sleeping), if he cries I repeat those steps to calm him down, it may repeat several times. It just takes less and less time to do those things. And not sure if it helps but I always say "It's time to sleep" each time I put him down.
For the last 2 weeks I wrap him when I see the very early tired sign (head turning, losing interest in toys, or is awake for 2 hours), then talk or sing to him for a few mins, kiss him and put in cot. He normally grizzles, blows raspberry or talks to himself for 10 mins then sleeps. If he cries I still pick him up for a cuddle, and if he can't settle in 30 mins I'll get him up to play waiting for the next cycle. Bed time routine is just bath, feed and sleep.
It's the resettling that we can't do yet, he mostly only sleeps for 1 cycle of 40 mins and wakes 3-6 times at night.