Starting at around 6 months kids start to find their voice. Parents should be able to distinguish between cries. As silly as it may sound, kids have a slightly different cry for everything. You should be able to determine when they are solely crying for attention. The idea is to slowly get them to be able to soothe themselves.
At nap time, feed the baby and get them to where they are just about asleep. Take them and lay them down at that point. Pat their back or turn on light music and get them to sleep. If you wait until they are already asleep you risk waking them up. Also, they will start having trouble falling without being held. When they wake wait a minute or two before heading in. If you go rushing in immediately it's teaching them that crying gets mom to come rushing in.
Starting a bedtime routine around this time makes the process easier. Start by doing the same process every night. This is hard when you have to be away from home or when you have company over. My routine was the same for all three kids. I fed them their evening meal, by this time they were on rice, gave them their bath and a bottle. If they wake in the middle of the night, I would change them, feed them and back to their bed they went. This is the same, wait a few minutes before running to them.
If you have those times where they are fussy and nothing is wrong, this is where the hard part comes in. You feed them, change them and pat their back until they fall asleep. Then, as soon as you leave the room they are wailing. Let them cry for a few minutes before you go back in. I'm not talking about letting them scream their heads off. I'm talking about giving them a few minutes before going back in. They will start to settle on their own. If after 5 minutes they haven't calmed, then you go in. After a few times they will start to learn to calm themselves faster. If you believe that something is wrong, yes go in and check. If you know nothings wrong, wait.
I've read several articles on both side of this issue. (I'll have to put the links down below.) I've seen where there are so many people discouraging allowing your kids to cry. I honestly stand firm behind the opinion of teaching your kids to self soothe. They need to learn little bits of independence. Running every time a kid cries only teaches them how to get your attention. The cries stop being about something wrong and turn into ways to get you to pick them up. Do not get me wrong, I am all for snuggles and lovey time with your babies. When it's time for love and snuggles, go for it. When it's time for bed, it's time for bed.
If you have another opinion or something you wish for me to read, please comment below. I am always up for reading articles. I am always up for a good debate.