I think sleep training gets a bad rap because so many people look at it the wrong way. It has nothing to do with getting your child to sleep LONGER. If a baby wakes up hungry, wet, cold, hot, sick, or teething it's the parents job to get up and fix it. I would NEVER expect any baby, no matter how old, to sit in there room and scream through that.
The idea is to help the baby learn what the tired feeling is, learn that sleeping fixes it, and learn how to fall asleep on their own. To do it right you have to make sure everything is good first. They've been fed, changed, burped and are wearing appropriate clothing. And most importantly, they are actually tired. Wind down before bedtime and wait for them to show signs of sleepiness before leaving them alone in a dark room.
After that's all taken care of and you've done your sleep or nap time routine lay them down and give them a kiss. They might cry a bit but give it a few minutes before you go back in. If they're winding down, great! They should fall asleep soon. If after 5 minutes they're still screaming full blast go back in and comfort. Repeat but make sure you give them time to cry it out before going back in. If you repeat more then 3-4 times then they're probably not tired or something else is wrong. With my daughter we've learned that if we repeat and she still doesn't wind down she's not tired and we go play quietly for a few minutes until she starts rubbing her eyes or fussing.
We started this at about 3.5 months old. She's now 6 months old and when she's tired she rubs her eyes and grabs for me. If I don't take her directly to her crib (like if i have to change her first) she freaks out. She's learned that when she feels tired she needs her crib.
Sometimes at night she'll go down but then wake up 30-45min later. She used to scream but now we just here her rustling. I looked in on her one time and she just rolls over and sucks her thumb. She's learned that sucking on her fingers and finding a new position is how she can fall back to sleep.
To me that's what sleep training is all about.