If EXACT accuracy is not required at this time, then don't worry about calibrating it yet. The unit comes calibrated across the entire range of the VNA.
I assume you do have the required adapters to connect your antenna.
Connect antenna to CH0.
Go to the menu
Select DISPLAY. Select TRACE and turn off all traces except trace 1.
In the menu, select STIMULUS and set the START frequency of the range you want to measure. For example, if you want to adjust your aerial for 440 Mhz, set the START at 400M. Repeat for STOP. Say, 500M.
If you want, you can calibrate it now and save it as a preset.
Back in the menu, select DISPLAY-FORMAT and SWR.
Use the switch at the top to move the marker to the bottom of the SWR curve to read the SWR ratio.
That's it.
You can narrow the range if required.