There isn't really any “training.” It’s just sleeping.
You don’t need to train a human being to sleep because we are already designed to do so. It does take some time for a new-born’s circadian system to mature, but after 8 to 10 weeks or so, a baby is capable of long stretches of night-time sleep. As part of his development he will grow attached to the sights, sounds, and feelings in the environment at bedtime. If they change throughout the night, the natural stages of light sleep turn into full awakenings; if they stay consistent, he should sail smoothly into deep sleep again. He has gradually become more conscious and in control, with the fine motor skills to find fingers or thumb, and perhaps even the gross motor skills to roll into a favourite sleep position.
We don’t “learn” to sleep, because it’s programmed deep in the brain, but we do “learn” sleep habits. Babies can learn helpful ones, like self-soothing and falling asleep in their lovely cot in a good sleeping environment, or unhelpful ones, like falling asleep while being nursed, rocked or bounced and with other sleep crutches. The helpful habits let a baby’s natural sleep skills shine. The unhelpful ones create the pattern of baby reaching externally for aids to soothe him back to sleep — resulting in disturbances and you hearing from him throughout the night.
What I practise teaches those good sleep habits. The parents of babies who have followed the method in my book and used the Reassurance Sleep Technique always say that their children sleep for long periods and they sleep well. They also know how to get to sleep and what to do if they wake up in the night. These learned skills last a lifetime.