Independent naps

There are two scandanavian methods that work pretty well (I'd like to note, sleep "training" really isn't a thing in scandanavia, but there are things they do to promote good sleep hygiene:

1) "lufting" - open windows, make the room cooler before baby goes down for sleep - no matter the outside temperature. Air out the room. You can close them again when you actually put baby down, but keep the room itself a few degrees cooler. Our bodies naturally cool down for sleep, and this helps promote the production of melatonin (which is why you even as an adult might find it harder to sleep when the room is too hot - babies are the same).

2) "I magen" - translates to something like "ice in the veins." It means steeling yourself and resisting the urge to go to them the second you hear them stir. Of course, if baby is actually dysregulated and crying, you go in and comfort them, but don't jump to every little sound - pause, get a drink of water, use the bathroom. (1)

They need practice to be able to link sleep cycles, and this means being able to wake, stir, and then go back to sleep again. Allowing them a few minutes to try to do it, and then only intervening if you need to and in the most minimal possible way, will help teach them to do it themselves, and not depend on you holding them for regulation.