Lux is a measurement of the amount of light you receive. The typical recommendation is to use a 10,000-lux light setting at a distance of about 16 to 24 inches from your face. Don't look directly to the light. Place it on the side. Adjust the brightness since it will take some time to get used to it and raise the setting with time.
For most people, light therapy is most effective when it's done early in the morning, after you first wake up for 20-40 min. Have these sessions consistently preferable at the same time. It will work as an extra cue for your circadian rhythm.