The best way to do it is as follows:
Take 220-your age, and that will be your theoretical max heart rate. Multiply that number by .65, that will be your target heart rate for fat loss. Divide that number by 6, and you'll get the number of beats you should count every 10 seconds.

For example: I'm 20. My max heart rate is 200bpm. 200x.65 is 130. If I divide that by 6 I get roughly 21-22 beats per 10 seconds. So, when I'm working out, I can put my finger to my temple/wrist/heart and count the number of beats in 10 seconds, should be 21-22 (for me).