I went on Amazon and bought all the top books on baby sleep and development. I read through them all, as well as several blogs and sleep websites. I gathered lots of advice.
You shouldn't sleep train at all, before a year, before 6 months, or before 4 months, but if you wait too late, your baby will never be able to sleep without you. College-aged children never need to be nursed, rocked, helped to sleep, so don't worry about any bad habits. Nursing, rocking, singing, swaddling, etc. to sleep are all bad habits and should be stopped immediately. White noise will help them fall asleep. White noise, heartbeart sounds, etc., don't work. Naps should only be taken in the bed, never in a swing, carseat, stroller, or when worn. Letting them sleep in the carseat or swing will damage their skulls. If your baby has trouble falling asleep in the bed, put them in a swing, carseat, stroller, or wear them.
Put the baby in a nursery, bed in your room, in your bed. Co-sleeping is the best way to get sleep, except that it can kill your baby, so never ever do it. If your baby doesn't die, you will need to bedshare until college.
Use the same cues as night: cut lights, keep the house quiet and still. Differentiate naps from nightly sleep by leaving the lights on and making a regular amount of noise. Keep the room warm, but not too warm. Swaddle the baby tightly, but not too tightly. Put them on their back to sleep, but don't let them be on their backs too long or they will be developmentally delayed. Give them a pacifier to reduce SIDS. Be careful about pacifiers because they can cause nursing problems and stop your baby from sleeping soundly. If your baby sleeps too soundly, they'll die of SIDS.
Don't let your baby sleep too long, except when they've been napping too much, then you should wake them. Never wake a sleeping baby. Any baby problem can be solved by putting them to bed earlier, even if they are waking up too early. If your baby wakes up too early, put them to bed later or cut out a nap. Don't let them nap after 5 pm. Sleep begets sleep, so try to get your child to sleep as much as possible. Put the baby to bed awake but drowsy. Don't wake the baby if it fell asleep while nursing.
You should start a routine and keep track of everything. Not just when they sleep and how long, but how long it has been between sleep, how many naps they've had per day, and what you were doing before they slept. Have a set time per day that you put them to bed. Don't watch the clock. Put them on a schedule. Scheduling will make your life impossible because they will constantly be thrown off of it and you will become a prisoner in your home.
Using CIO will make them think they've been abandoned and will be eaten by a lion shortly. It also causes brain damage. Not getting enough sleep will cause behavior and mental problems, so be sure to put them to sleep by any means necessary, especially CIO, which is the most effective form. Extinction CIO is cruel beyond belief and the only thing that truly works because parents are a distraction. The Sleep Lady Shuffle and Ferber method are really CIO in disguise or Controlled Crying and so much better than Extinction. All three of these will prevent your child from ever bonding with you in a healthy way. Bedsharing and gentler forms of settling will cause your child to become too dependent on you.
Topping the baby off before bed will help prevent night wakings. When babies wake at night, it isn't because they are hungry. If the baby wants to nurse to sleep, press on the baby's chin to close its mouth. Don't stop the baby from nursing when asleep because that doesn't cause a bad habit. Be wary of night feeds. If you respond too quickly with food or comfort, your baby is manipulating you. Babies can't manipulate. Babies older than six months can manipulate.
Sleep when the baby sleeps. Clean when the baby cleans. Don't worry. Stress causes your baby stress and a stressed baby won't sleep.