As a matter of necessity I needed him out of our bed and into his cot but couldn't transfer him down asleep so "sleep training" was non negotiable for me at that point - I know people feel strongly about not doing it under 6 months but I just couldn't cope with another 2 months of co-sleeping and constant wakings all night - I was loosing it.
I tried PUPD, the picking up soothed him "in the moment" but did nothing for the root cause of his upset which was not being able to go to sleep - which in turn infuriated him and made him the most distressed I've ever seem him. He's not a very "cry-y" baby and this was a particular low point for me.
So I decided instead of keep picking him up and seemingly annoying him more and more that I would make sure he was comfortable, dry, etc...then just put him down and leave him.
I was in the room at the head of the cot the whole time, but out of his sight. At intervals (not particularly timed) I'd lean in, re-position him if necessary, shush him and whisper to go to sleep and put my hand on his chest for reassurance then back off again.
After about 30 minutes he found his two middle fingers to suck on, and within minutes he'd soothed himself to sleep.
The crying was never a proper distressed cry (like it was when I was picking up and putting down!) but more of a frustrated, tired grumble.
Within 3 days we were doing all naps in the cot and I was able to pop him in in his grobag, he'd pop his fingers in and drop off to sleep with no fuss.
Since then we've had a bit of a regression and had to repeat the process. He's dispensed with the fingers now and instead rolls over to his tummy and does a low groan before dropping off.
I stand by it as one of the best parenting decisions I've made. We went from waking up every 2 hours all night needing a boob in mouth to go back to sleep to leaving him to self settle when he stirred and only feeding him when he got grizzly which is usually only once now around 3am.