If you open the meter there are calibration pots for the high and low power and swr but I cannot find any instructions anywhere on how to calibate
You'll need 1:1, 2:1, 3:1 dummy loads (very close in resistance and capable of handling the transmitter power). You'll also need an RF wattmeter that has at the very minimum a 4:1 ratio of maximum error compared with the meter you're calibrating. Meaning that if the manufacturer of your meter says it's plus or minus 10%, the standard must be plus or minus 2.5% or better.
Dummy loads and wattmeter must themselves be calibrated (otherwise you don't know what's right and what's wrong).