The first thing you have to do is to twist the tubes or barrels of the pair of binoculars around its central axle until the two eyepieces fit exactly on your eyes and you can see a circular field of view. This is called adjusting the eyepieces to your interpupillary distance (IPD).
Now you can continue to calibrate the instrument to your eyes in the following way (assuming the diopter is in the right eyepiece):
Step 1: Put the lid on the right objective lens and then focus on an object some 10 meters away with the left eye; adjust the central focus mechanism until the image is perfect.
Step 2: Now put the lid on the left objective lens and, then focus on the same object looking through the right eyepiece, this time adjusting only the diopter (do not touch the central focusing mechanism) until the image is perfectly in focus; lock in the setting if your binocular does have this facility on the diopter.
Now the binocular is calibrated to accommodate the difference in your eyes (and your eyes only) and you can enjoy your instrument, adjusting only the central focusing mechanism. It is recommended that you do re-calibrate it from time to time, since it is possible that the setting on the diopter might change for whatever reason.
If you allow another person to use your binoculars, he actually has to calibrate is to his eyes first (in which case you have to do the calibration all over again for yourself!).