They work in a similar way to HPTs. They are a urine test, usually you collect your urine in a cup and then you either dip a strip into it, or drop some urine into a little 'cassette'. You can get mid-stream tests, but they are less common, and more expensive. Two lines +, one line -! You usually need to do a few each month, starting from just before when you think you will O.

I have tried a few brands and found them all to be good, but by far the cheapest are the lullaby conceptions (ad at top of this screen) which you purchase online. Otherwise you can get them at the chemist, priceline, supermarket etc. I think Fortel and Confirm are also good. (But more $$$)

Good luck! Ask away if you have any more questions!