It's easy. You just have to be diligent about taking your temp the same time every morning BEFORE you do ANYTHING. Have thermometer within reach of waking, take your temp, then you can get up and move around. It is really important to get the basal body temp.
Mark your temp on a chart. You can make your own or use a fertility website like Fertility Friend or software like Ovusoft.
Your fertility cycle has two main hormones... Estrogen (the cool hormone) and progesterone (the warm hormone). At the beginning of your cycle the cool hormone (estrogen) is working to make your egg mature and following ovulation the warm hormone (progesterone) maintains the luteal phase for implantation.
So what you are looking for on your chart is a .2 degree higher temp than any of the prior 6 days. This increase in temp must be maintained for 3 days to show ovulation has occured. (Increase in temp shows that progesterone- which is produced by the ruptured folicle- took over for estrogen).
The day prior to the temperature shift is the day you ovulated. So by charting bbt, you can only determine when you've already ovulated. But you can use it to predict ovulation in the future by charting several cycles and seeing if you can find a pattern. You can also cross check your bbt with the other primary fertile sign which is cervical mucous. This fertility sign starts 3-5 days prior to ovulation and ends when ovulation has occured.
Both of the fertility charting options above use smart technology and figure out when you've ovulated for you.
You can get loads of information from both websites above, and they will tell you all the information you need to get started. It sounds overwhelming at first, but it really is very simple, and you can gain a whole bunch of information about your health by charting. (You can tell if you have a sufficient luteal phase, if you ovulate every month, maybe if you are pregnanct if your body shows the 'implantation dip' etc. etc. etc.)
Good luck!
