whats the best way to download workouts to a garmin 520 to use on a Wahoo kickr?

There is a very easy way to do this. Simply plug your Garmin into your computer, open the workout on Xert Online, click the TCX icon on the top right and save the TCX file to the NewFIles folder on your Garmin.

Then, when you unplug and start the 520, go to Menu (down button) and selecting Training and then Workouts. Your workout will be listed there.

To use the Wahoo KICKR, be sure to have the latest firmware installed and configure it on the 520 as an ANT FE-C device. The Garmin workout will automatically control the resistance on the KICKR.