You put a bottle cap full of salt, , put several drops of water on the salt, not much water at all either. 3-4drops of water.
Put all your hygrometers in a airtight mason jar, or big zip lock bag, it just needs to be airtight, and wait several hours, they say wait 6hours, but alot of times it only takes 3-4hours.
They should all read 75percent, by that time, then you just mark your hygrometers if they are not reading 75percent.
Example if one hygrometers reads 73percent ,I put a small piece of masking tape on it ,saying plus 2, if one reads 78percent, I mark the tape minus3.
I ain't good at explaining, but all you have to do is Google How do cigar shops calibrate their hygrometers, an they can explain it lots better than I can.
I recalibrate mine every 3-4months, an they are usually right on the money, everytime, I hope that helps some 9fingers, like I said I ain't good at explaining.