I’m confused—that sounds like the G5 start up. “No calibrations” is one of the primary selling points of the G6. Are you sure you’re not looking at a G5 manual? As long as you enter the sensor code the G6 wont ask for a calibration, so that’s why you’re not finding much about it. If you do need to calibrate, use the “Settings” link in the lower left of the app’s home screen, and you’ll find the Calibrate option toward the bottom of the settings list.
You’re right that the readings can be erratic in the first 12-24 hours, and opinions vary on whether to just wait that out or try calibrating. The claim against doing it is that calibrating resets the algorithm and you get better results overall if you resist, But I will get impatient and do one if it’s far enough off from my Contour.
One thing that seems to help is to “pre-soak” the new sensor by inserting it 12-24 hours before you do the transmitter switch-over. I.e., you’re wearing two sensors, the old one that’s still running and the new one that you haven’t swapped the transmitter into yet. The idea being that this gives the filament more time acclimating to your interstitial environment and any inflammation around the insertion has time to settle down. Seems to make some difference for me, but I still get some erratic readings at the start of a session anyway. Lots and lots of comments on this if you do a site search on it.
ETA: @Firenza describes the “Settings” link being to the upper right in the App, so it may differ according to what your smartphone platform is. Mine is an iPhone.