It is recommended to do the test twice a day (one after 10am and one around 8pm). First morning urine is not recommended since LH surge may happen later that morning. When you test twice a day you have higher chances not to miss your LH surge.
Liquid intake should be reduced approximately 2 hours prior to testing (no more than 8 oz during these 2 hours).
Another important thing is that for some women a clear positive result may last for a couple of days, for others it can be less than 12 hours. That's why we always recommend testing twice a day so that there is a better chance to catch the surge.
Please note positive LH test indicates that the ovulation will take place between next 24 and 48 hours.