You must chart at the same time every morning or at first wakeup. In the beginning I would suggest any woman doing this should chart for 6 months while always using the protection and doubling up with condoms during the fertile time. You don't need to have pain and agony from contraception. You can at least have peace of mind that you know your method of protection is not possibly injuring you too. If your religion prevents the use of other forms of protection, and you can't see yourself with a baby right now, please abstain from sex.
Abstinence is, after all, the most effective form of protection.
So far, it's only available in the UK and Europe because the FDA in the United States hasn't approved it yet, but it may be here soon. What you buy is a small computer and urine analyzer. By giving it a few more samples now and then, it can stay on track. Here's a website to learn more about Persona. And here are a couple more links to sites that describe natural methods of contraception.