There are some frequently asked questions and answers after the general instructions.
Charting your BBTs is really pretty easy. Basically, what you are doing is taking your temperature first thing each day and plotting the temperature on a chart. What you are looking for is to see a shift of at least .4 degrees Fahrenheit after ovulation making your chart biphasic (showing low temperatures before ovulation in the follicular phase, and higher ones after ovulation in the luteal phase).
Take your temperature first thing in the morning before you get out of bed or even speak -- leave your thermometer at your bedside within easy reach so you don’t have to move much to get it. If you use a glass thermometer, make sure you shake it down before going to bed.
Try to take the temperature at as close to the same time each day as possible -- set an alarm if you need to. Staying within a half hour either side of your average time is a good idea because your temp can vary with the time (i.e., if you usually take your temperature at 6 a.m., it is OK to take your BBT between 5:30-6:30, but the closer to 6 the better). The normal variation is by up to .2 degrees per hour -- lower if you take your temperature early, higher if you take it late.
It is best to take your BBT after a minimum of 5 hours sleep, and at least 3 in a row is preferable.
You can take your temperature orally, vaginally, or rectally -- just stay with the same method for the entire cycle.
You should try to place the thermometer the same way each day (same location of your mouth, same depth vaginally and rectally).
Plot your temperature on your chart each day, but refrain from reading too much into it until the cycle is done.
Some women, not all, have a temperature drop when they ovulate. If you see this drop, it is a good idea to have sex in case you are ovulating.
What you are looking for is a temperature shift of at least .4 degrees over a 48-hour period to indicate ovulation. This shift should be above the highest temperatures in the previous six days, allowing one temperature to be thrown out as inaccurate (fluke, illness). Perhaps the best way to explain this is to show an example.