First you need to calculate the power output of your signal generator using the power meter. Collect data for the frequency points of interest to you.
Next you need to calculate insertion loss for the coupler. Attach the input of the coupler to the generator, the power sensor to the output, and a termination to the coupled port. Record data for the same frequencies that you collected as data, save this for calculating the coupling factor. Then subtract this from the data you recorded previously to determine the insertion loss of the coupler. Report this data on your calibration report.
Now move the termination to the coupler output and the power sensor to the coupled port. Measure the output at the coupled port for the same frequencies, and subtract that from the data you collected earlier to calculate the coupling factor. Record this data on your calibration report.