How do I do a major calibration?

Calibration - that is, adjusting the scale so it weighs accurately over its range - depends on the model, so you have to read the instructions. It probably requires using a "standard weight" which you can easily buy online but need to keep clean. As a simple check each time I use it, I wrote its weight on each pot I use for samples. So I switch on the scale and it reads 0.00. Put the empty pot on and it reads the number on that pot. Then press "tare" to get zero before weighing a sample. If the empty pot doesn't weigh correctly, I need to calibrate the scale.