Throw away the powdered calibration solution mix that comes with the meter. Unless you have lab quality water measuring equipment, you'll mix up a cal solution that can't be trusted.
Immerse the tip of the meter in General Hydroponics pH 7.0 calibration solution, then rotate the calibration screw back and forth until it reads 7.0.
You'll quickly learn how often you need to check the calibration. About once a week should do it.
Rinse the tip after every use. The glass bulb in the tip is extremely thin (so it can sense the electric charge gradient), so be gentle with it.
Keep the tip wet, always, to prolong its life. Ideally you store the tip in potassium chloride storage solution or just do what they do in most labs and leave it immersed in pH 7.0 cal solution.
There are good videos and other info about pH and pH measurement. I recommend learning all you can about pH. It's not terribly complex, but there are subtleties (like the scale is logarithmic, so ph 4 is 100 times more acidic than pH 6, for example).
pH meters are pretty simple electronically, but they have a hard job to do. If you don't use them carefully and keep them calibrated, you're better off using pH color drops from the pet store. Too many cannabis growers are getting junk data from their meters because they don't know what they're doing and don't take care of their equipment.
