Well, I didn't switch my son to his crib until 5 months. We co-slept with him swaddled before or he would nap in his sling or swing. I started the switch when he was going to sleep reliably at night and only waking up once in the early morning to nurse. I created a bedtime routine: diaper change, pajamas, book, get swaddled, nurse, bed. Then I let him cry for five minutes then I would get him out and put him to bed the old way in our bed. After 4 nights, he fell asleep in the crib before the 5 minutes were up. But then he didn't the next night. Since I had decided that having slept in his crib one night he needed to continue, I started letting him cry for up to 15 minutes. If he was still crying, then I would go in, nurse him some more and then try again. He always fell asleep nursing either the first or second time so we didn't have a problem until I began losing my milk supply. I cut out the nursing and instead gave him a bottle before the bedtime routine. After a week or so, he reliably fell asleep within the 15 minute time and now at 9 months he almost never cries at bedtime. We also don't swaddle him anymore since he was 6 months and started getting out of it in his sleep. I didn't try to get him to nap in the crib until after we dealt with bedtime because he was a really bad napper. Now he naps in his crib, but if he doesn't fall asleep after fifteen minutes we take him out and try again later.
Sorry so long. Hope it helps.