You need to place the EC probe and the pH probe in the solution together. The EC probe also takes the temperature of the solution. The EC of the solution will indicate to your meter that it is calibration solution and it should actually "auto calibrate."
Mine, when set to calibration mode, will ask for the first solution. When it gets it, then it just asks me for the second solution. I don't need to tell my Hanna GroCheck combo meter "this is 7.01 calibration solution" because the meter recognizes it as such. The EC probe is a vital part of doing that.
Ensure you clean the probes before calibrating and while calibrating. A quick spritz of water will do the trick.
Then you'll want to pay close attention, and record everything, as you add your nutrients. It should be predictable every time. The water should start at one value. Xml of the first nutrient drops the pH to another value. And on and on. Watch the pH every time and this will let you know when it is important to calibrate again.
I got my meter about 3 months ago. Calibrated it once since then and it didn't need it. They stay remarkably dialed in even when used in a hydro-organic system and getting a little "slimy."