well, I'm sure baby whisperer explanation is better than mine (try library or charity shop, or friends etc) but...
basically she advocates putting them down to sleep while they're still awake. There is however a 'window' of opportunity when baby is kind of geared up for sleeping, and you need to recognise the signs so you can go for it then ... but I can't recall what they were- I just know what my dd does when she's tired IYSWIM.
She uses a 'routine' but without timings, just whats right for your baby- EASY- Eat, Activity, Sleep, You time. So that they eat when they wake, not just before. This is so they don't associate food with sleep, and expect to be fed off to sleep IIRC.
Pick-up, put down is where you've put them down to sleep, left them and they're crying, so
go back to them, pick them up. Pat their back, saying 'shh shh' only for 30 secs or so, so they calm down. Apparently babies cannot concentrate on the patting and shhshing at the same time (ie more than 1 thing) so they calm.
Put them back down, leave room.
When they cry again, go back, pick them up, pat them, 'shh shh' etc.
Repeat ad infinitum.
When they wake after 20/30 mins etc, and cry, do the same thing, till they go back to sleep.
With a slightly older baby (8mo??) she says, leave them lying there, and pat their back saying 'shh shh'. The patting thing really helps- in fact dd does this with her cuddlies now when she's playing at putting them to bed , so it obviously stuck!
There are bound to be other threads on this in 'sleep' if you search, and I think there's a baby whisperer website.
We were in an awful state with our dd (turned 2 this month). She could not sleep anywhere. She did however have reflux which caused her lots of pain. There were days when she could not remain asleep for longer than 5 mins and it was taking 1hr each time to drop off- exhausting, infuriating, overwhelming for us and for her. The problem was that we couldn't tell whether she was in pain, or angry at not sleeping, so we had to crack the sleeping problem IYSWIM.
She claims it will take 3 days with 5 mo old, and about 5 with a 8/9 mo. We took 3 days, and it was hellish, but we did it. She now sleeps 13 hours a night, and naps 1-1.5 hours after lunch, which is pretty good for a 2 yr old- though she has always needed a lot of sleep.
If you think there may be reflux, you could try raising the head of her cot (eg on thick books)- this made a big difference to the length of time dd could stay asleep for, and meant that once she could resettle herself she would have proper sleep- and so could we!
What also made a difference was switching from blanket and sheet to sleeping bag/grobag. Even with all her thrashing/faffing she was still toasty enough.
I do think how you feed them makes a difference- the most you're going to get a bf baby to sleep at 5 mo is 4 months, most will do 3 max. I think dd switched to 4 hour 'routine' at around 7 or 8 months- made a big difference to my sleep! I also cut down to 1 feed per night around then, so she was sleeping in 6 hour blocks or so.
Hope some of this helps! I really feel for you, those days are prob a big part of why we still only have 1 child! It will pass, I promise.