i used the videos from the DJI website.
that said here are the calibrations and the order in which i would do it...
1. firmware updates via the DJI GO app.
2. IMU calibration in the DJI GO app -> settings/drone/advanced/imu/imu calibration (make sure to do on a flat surface like a floor or table top, more level the better, the calibration has on screen instructions you can't screw up.)
3. controller calibration in the DJI Go app -> settings/controller/calibration (to make sure there are no inaccuracies in the sticks for your controller)
4. gimbal calibration in the DJI GO app -> settings/camera/gimbal-auto calibration (do on a level surface again, indoors)
5. gimbal calibration (manual) in the DJI GO app -> settings/camera/gimbal-roll calibration (adjust from what the auto system did until the image looks level)
6. compass calibration in the DJI GO app -> settings/drone/advanced/compass/compass calibration (self explanatory again but this one should be done outdoors, away from large metal objects like cars while hand-holding the drone about 4-5 feet above the ground.)
all this sounds like a lot but the only one I do often is the compass calibration and even that isn't totally necessary each flight. anytime you move the drone a long way before flying it again it might also be useful to redo the IMU calibration. gimbal calibration should only be needed when the horizon doesn't look level again.
now i'm a new drone pilot so perhaps i am missing a nuance somewhere but the above was what I did.