What happens next?

[SES84]

Hi ladies, I hope you can help me. I had my AMH scan last week and at the appointment I was advised that the next steps would be to call the clinic the day I start my period. I know that you generally start taking the down reg drugs etc. on day 21. It was wondering what will happen between the day I call the clinic (day 1) and when I start my down regulation? (day 21). My last appointment felt a little rushed and I came away with lots more questions! Any advise/info would be great (1st ISCI cycle) xx