I'll recommend you to begin now or next month.
I did it with my eldest son when he was 8 months since
he was born premature baby and thank God it works well.
Now with God's help I'm going to do that with my twins
next month.In the book of Tracy Hogg "The Baby Whisperer"
solves all your problems she recommended around 7 months
and it has to be done in 3-day incrrements, to insure that your
baby makes up during the day what yor're taking away at night.
If you need more help you can send me e-mail and I'll happy to do that.