(I got this from babycenter)
Depending on the brand of urine-based OPK, you’ll either collect your wee in a cup or hold a stick under you as you wee. Coloured bands will appear on the test card or stick to indicate whether or not the LH surge is occurring. Digital OPKs use symbols, such as a smiley face, to tell you when you are on your most fertile days.
Instructions may vary slightly depending on which kit you use, but in general you should try to collect your urine between 10am and 8pm. The optimum time is supposed to fall between 2pm and 2.30pm.
Try to collect your urine at about the same time every day, though this is not absolutely necessary. Don’t test your urine as soon as you wake up, because you may miss the first day of your LH surge. Try to reduce the amount of liquids you drink for about two hours before you do the test. Too much liquid will dilute your urine, which could make it more difficult to detect the surge.
Read the results within 10 minutes for the best results. A positive result will not disappear, but some negative results may later display a faint second colour band. So throw the test card away once you’ve read the result. Urine-based OPKs generally provide five to nine days’ worth of tests.
Using a salivary ferning OPK is perhaps easier, as all you have to do is place some of your saliva on a slide by using your finger or licking it. Do this first thing in the morning, before you’ve had anything to eat or drink. Make sure you don’t put too much on and that it is free of any air bubbles.
You then wait for the saliva to dry and then use the microscope to see whether there is any ferning or not. Compare your slide with examples in the instructions to see whether you’re in your fertile window.
With either type of OPK, it helps to work out when your fertile period is likely to start. It’s usually the length of your usual cycle minus 17 days. So, if you have a 28-day cycle, start testing on day 11 and carry on for six days. Use our ovulation calculator to help you work out when your fertile window is likely to be.