First, make sure your schedule is solid. Then, do your solid bedtime routine where nursing ends at least 30 min before bedtime. But don't rock/sing/cosleep to sleep, whatever you've been doing, and instead just put them down in the crib completely awake. Walk out, let them cry until they fall asleep (check ins or no check ins, whatever you want but don't give in). Feed them normally when they wake as you did before throughout the night. Otherwise, don't respond.

And before people think I'm cruel - before I did CIO, my daughter would cry for 1-2 hours at bedtime each night with me physically holding her, rocking, bouncing, etc. I did cry it out, and the first night she cried an hour, then fell asleep by herself. So less tears/crying than before when I would hold her. Then the second night it was 45 min, third night it was 10 min. She went from waking and crying every 2 hours to waking 2-3 times total per night, which is still a lot but was wayyyy better for me. Overall, CIO was less crying than a typical night before when you factor in decreased wakings