Tune your radio to WWV at 10mhz, 15mhz or 20mhz. The higher the frequency the more accurate the adjustment will be.
Listen on AM to WWV and if it is transmitting a tone do the following, with the radio display on the exact frequency such as 20.00000 for example, flip the mode switch between USB and LSB and listen to see if the tone is the same. If it isn’t, adjust the main Reference oscillator so that the tone transmitted from WWV is the same.
You can also switch to AM and verify that the tone is the same as USB and LSB. That’s it. Your radio is set on frequency.