The way to calibrate are simple read data from HX711 without any weight this data are your Zero weight "0.000"
Now add a known load on your cell like 1 kg or 1 Pound or 1 Brick or whatever you want to measure in you will get a new value
This new value - the zero value are now = to 1Kg or what you want to measure in
Now it pressy simple math to convert any value to a weight
I assume you use Arduino, but if not it's the same way to do it
Have a look here how to connect weightcells to the HXD711
https://learn.sparkfun.com/tutorials/load-cell-amplifier-hx711-breakout-hookup-guidein the bottom there are links to 2 software, 1 for calibrate and 1 for readout
If you run the calibrate software you can inc / dec the calibration value so your weight fits the actually load
Hope it helps