I feel like I am a bbt expert now lol.
If you are temping orally, you need to place it under your tongue towards the back (like taking a temp for illness). It most definitely needs to be same time every day (give or take 20 mins or so). You also need to do it as soon as you wake up. Don't even move yet! I place my thermometer and alarm under my pillow (it's a vibrating alarm on my mobile) so when I sleepily go to press the button to turn it off, the thermometer is there too.
I often doze off with it in my mouth but it doesn't matter because once it's reached its temperature, it stays like that. My thermometer actually shows you the last reading for about 3 secs when you turn it on next time.
Don't take into account for any later readings. Sometimes your temp goes up, stays same or goes down. It varies on weather, what you did and your time of month.
If you wake up late, take it anyway but know that it could be slightly off.
If you wake up earlier than anticipated and it's less than 3 hours to go till you actually get up, take your temp then and at your normal time. Take note of the difference. There's a website called whenmybaby which has a calculator on there that works out what it probably would've been.
You need to have been asleep for 5 hours with the last 3 hours unbroken. X
