The kits don't last forever. They have a shelf life of about two years, so be sure to check the expiration date on the box. Prices range from about £8 and £22, depending on the brand and the store. Most come with a five day supply of test sticks. Your job is to urinate on the stick and then wait for a line to appear. The instructions show what colour/shade indicates that ovulation is imminent (within 24 to 48 hours). If it’s too close to call, retest within the next 12 hours.
If you're having a difficult time with the kits (is it the exact colour that is on the box?) you could try one of the small electronic monitors. It reads the results and can tell you even earlier when to expect a surge in LH. However, take into account that these monitors are expensive and then decide whether it is worth it to buy one or not.