R.C.
You are going to get so many different answers but this is what I did and it worked:
1. Set aside 3 days/nights where you can plan on being in hell. Block them off on your calendar.
2. Get a big bowl of candy to help deal with the anxiety because it is heart breaking to hear your kid cry endlessly.
3. Put her (or him, I don't know what you have) to bed and let her cry for 5 min, go back in after 5 min but DO NOT take her out of crib. Give her a kiss and leave. Let her cry for 10 min, go back in, give her a kiss, do not take her out of the crib and leave. Let her cry for 15 and do the same. Then either try for 20 minutes or just let her cry until she falls asleep. Do this for 3 nights in a row. It is not easy but in the end it is SO worth it. She will not remember it, she will not be mad at you in the morning. You can do it!
Note: For my older one, it took 3 nights. For my younger one, it only took 2. Stay strong. Get candy!! Good luck. :)
~R.