Hello Remember :)
Your bbt rises after ovulation. From day 1 of your cycle, you need to take your temperature first thing in the morning. It needs to be taken before getting out of bed, before speaking, drinking, etc or it won't be accurate. It should also be taken around the same time each day and after 3 hours consecutive sleep. You then record your temperature (I think most of us here use an app on our phones called fertility friend to do this and I would recommend you do the same). If you are ovulating, you will see a pattern of lower temps before you ovulate, and after you ovulate your temps will rise and will stay high til AF comes (many people get a sharp temp drop the day before or day of AF so we know it's coming).
You can't use any old thermometer: you need a basal body thermometer as that calculates the temps to two decimal places. The rise in temp can be quite subtle so an ordinary thermometer won't detect the difference much. For example my pre O temps are around 36.32 and post O temps are around 36.63. You can buy one of these thermometers cheaply on Amazon.
I set my alarm every morning so I take it at the same time but if you wake up early or late or have disturbed sleep don't worry too much, as long as it's not happening every single night you should still be able to detect your O pattern.
Most ladies temp orally, I temp vaginally as I get a much clearer more consistent pattern that way.
Once you're charting you can see a) if you're actually ovulating and b) the exact day you ovulated, as it will be the day just before your temperatures rise. So you can't use it to plan when to DTD, other than when you've been doing it a few months you may find you ovulate around the same day each month. But you'll be able to see how well you timed DTD that month, and also how long your luteal phase is (a short luteal phase of less than 10 days can be an issue).
Sorry for essay, shout if any questions!!