Most women start discussing this with their doctor or midwife around 7-8 months pregnant.
We are doing a series in my Natural Birth group here at MedHelp. While the series is geared toward natural birth perhaps the advice and information can help you to piece together your birth plan?
http://www.medhelp.org/forums/GroupTrack-We-Want-a-Natural-Birth/show/1071?camp=msc
On the final post of the series (post 9) we'll piece all of ours together and share them.
Otherwise here is a basic definition of a birth plan - a birth plan is merely your wishes and desires for your birth. You should include information for labor, delivery, monitoring, pain relief, in the event of emergency or cesarean, care of baby after birth.
A birth plan is actually more or less your wishes. When someone says, "Be mindful your birth may not go as planned" we have to realize that you are not actually *planning* the birth but expressing your desires. To plan a birth would be to say things such as, "I will only labor for 3 hours" and "I will push for 5 minutes". We're not planning the birth in that sense, just expressing our desires.
Not having a birth plan or discussing options with your doctor or midwife will most likely result in "default procedures". If you don't want to be defaulted it is important to express your desires.