Start the morning after it gets there. Be prepared to wake up at the same time every day to temp and have the thermometer and a notepad on your bedside table so you can temp before you do ANYTHING else. Don't get up or eat or drink anything before you temp. If it's hard for you to hold your FMU long enough to temp (as it is for me lately) you can temp, then go pee and temp again before you do anything else. If the temps are consistently the same, you can temp when you pee. Be forewarned, no matter how much you read about BBT tracking, You will probably freak out every time your temp changes for a while until you know your temp cycle and are more comfortable temping, I know I do. GL with it!!