Test at the same time every day. Either morning or afternoon is okay, but the afternoon (between 12pm-6pm) will give you the best chance of catching your LH surge.*
Your LH surge can occur within a short 12-hour window, so testing twice a day will increase the likelihood of catching it. If you choose to test twice a day, test once in the morning, drink fluids as you normally would, and plan to test again in the afternoon or evening — just avoid urinating for 2 hours before testing again.
*In June 2021, we updated our recommendation for the best time of day to test. Previously, we recommended testing in the morning. Based on the data we've analyzed over the last year, we've updated this recommendation to testing in the afternoon. For more info on why we made this change, check out this post on our blog.