I Am Going For Frozen Embryo Transfer Next Month. What

I am going for Frozen Embryo Transfer next month. What is the procedure with using GNRH and birth control pills. I am not very clear on this from my doctor even though she explained me twice. She told me to start taking Birth Control Pills from about last 3 weeks and and continue till my next appointment which is on 29th of September. I just got my periods today and want to know when I will be given GNRH injection to suppress my periods and for how long I have to continue taking BCP. and how long the procedure will be from my next cycle 1 day.