How do I teach my 6 month old to self sooth himself to sleep?

The best way I've seen, that seems the most gentle, is to start associating other things with bedtime. If you're rocking him to sleep, start a few other habits, too.

Play soothing music, or use a specific blanket to hold him in, or something else that he can keep with him when he's going to sleep. Then after a week or more of this, you slowly rock a little less and let him use the other items to help fall asleep. It would take at least a month to do, the way I've heard it described.

Although at 6 months, there are still many infants who don't fall asleep well. It's pretty natural, as much as our culture acts like it isn't.

wait until he's properly tired before trying to rock him to sleep. A lot of babies will fight sleep if they're not really tired, or just want to play, or you've missed the signs that they're tired and they get overtired because a lot of babies are pretty particular about what they need to fall asleep comfortably. Once that happens, the best thing is to let them play, and talk to them, keep the room quiet and don't overstimulate them, but just play with them, let them sit up and talk to them and don't let them know you're trying to get them to sleep. Then when they start showing signs of being tired and start getting grizzly try it then. No baby will self soothe to fall asleep and remain healthy. All that happens is they get so tired they can't fight it any longer, or get so miserable and upset they stop trying and start withdrawing from people. When you put them to sleep gently babies tend to fall asleep quite quickly once they need sleep and their sleep routine has happened, like they're getting fed, have a blanket or pacifier or even just a cuddle. Emotional security plays a big part in keeping babies content and relaxed.

Most babies under a year old go to sleep nursing, or taking a bottle, while being rocked. It's normal, it's right, and it's the most comforting for them. A six month old baby should not soothe himself! To try and 'teach' him to is unnecessary at best, potentially dangerous at worst.

The happiest, healthiest toddlers are those who co-sleep with their parents until they're at least two or three, always able to nurse themselves back to sleep.

If a baby does not fall asleep while feeding, but seems tired, then you could try putting him in a stroller and going out for a walk to help him get to sleep.