Let me give a super-basic run-down of charting for the newbees: (most material from Taking Charge of Your Fertility (TCOYF) by Toni Weschler, MPH)
The goal of charting is to figure out when you ovulate. The only problem is that charting only tells you after. So you need to BD plenty before ovulation occurs...
First, you need a BBT thermometer. Mine is the pink one from Target. Make sure it is a basal thermometer. My thermometer beeps when it's done and saves the temp, so I can take it with my eyes still closed and go back to sleep.
I've heard mixed reviews about this thermometer, but it's worked well for me!
Take your temperature at the same time every day, before you have gotten up in the morning. It's important that you've had at least 3 hours of uninterrupted sleep for your temperature to be accurate. Checking your email first, or going to the bathroom will raise your temp. I take my temp at 5:30 a.m. because that is the earliest time I get up. On the weekend and in the summer I still take my temp at this time. It's early enough that I can still fall back asleep.
Tips on taking temp. You can do this orally or vaginally. I take mine orally. If you take it orally, the most accurate reading is toward the back of your mouth, under your tongue. Just make sure you use the same spot each day. Sometimes mouth breathers get wacky temps. If this happens to you, maybe try taking it vaginally. I have no advice to offer about this because it scares me! : )
Next, record your temp. You can keep your chart on paper, but I suggest using an internet or smart phone app because the tools are extremely helpful in interpreting your chart. I use the online charting on TCOYF.com. The site is slow at times, but I prefer the layout. Another popular site is FertilityFriend.com. There are many of different settings you can use. When I have questions I ask the true experts in the forums on those pages.
Last, interpret your chart. This is the tricky part! Here's my chart this month for an example.
In a nutshell, the temperatures in the first half of your cycle are lower than the second half, after you ovulate. You are looking for your temp to rise above your coverline and stay there for at least three days to confirm that you have ovulated. Usually it's a .5 increase or more. Your temp should stay about the coverline for most if not all of the second half of your cycle and the will fall in increments or in one big drop the day of or a few days before your period starts.
It's kind of like magic. How many times have you wanted to not be surprised by your period. I can literally say, "My period is going to start today!"
There are many more nitty-gritty details, but I'll leave them for a future discussion.
One last bit.
There are many things that can affect your temp including: alcohol, sickness, mouth breathing, electric blankets, and many, many more. Most of these will increase your temp. For myself, if I have one drink, I see almost no effect, but if I have had many, my temp will be up to .5 higher than normal. When you know a temp is out of the ordinary, you can "disturb" it on your chart and it won't be used to calculate anything.
I hope this has helped! If you have any questions, leave them as comments!