How to set up a Kickr Core to work with my Garmin, dynamically change "elevations" (resistance), etc I recently got a Wahoo Kickr Core indoor direct drive trainer. I already have a Garmin Fenix 5X and a Garmin 820. (I use Training Peaks for my workout information.) I have no idea what to do now. I guess I want to connect them probably using ANT+. Do I want to connect both Garmins to the trainer, or just one of them?

How do I change the resistance? Do I use the Wahoo App on my phone or one of the Garmins? Is there some way to make the trainer simulate one of the rides I did last summer based on data saved somewhere in one of the Garmins?

The goal is to find some way to ride the trainer and have something simulating a "real" outdoor ride: hills, changes in resistance, etc. I'm not at all sure how to do this, or if it's even possible. I don't really want to subscribe to Zwift or Trainer Road at this time. I'd rather just get on the bike and feel like I have the ability to control the resistance or that it's changing it in some logical way. 

Are there Youtube videos, a website, or something similar out there I could learn from. My searching has found lots of videos on how to connect, but not what to do with them after they are connected.
Yes, you can do this with your Edge 820.

You need to pair the Kickr Core to the Edge 820 using the sensor type “Indoor Trainer”.

Menu > Training > Indoor Trainer > Pair ANT+ Bike Trainer

You can “re-ride” a previous activity recorded on your Edge 820 indoors - the Kickr Core will adjust its resistance based on the elevation recorded in the activity.

To do this, choose

Menu > Training > Indoor Trainer > Follow an Activity.

If you don’t have a ride you want to repeat indoors, you can create a course in Garmin Connect and use “Follow a Course” in the Indoor Trainer menu.

This web page at Garmin support gives the steps to connect the trainer to the Edge 820.

This web page at Garmin support gives the steps to follow a course or activity on the Edge 820.

This video from Garmin shows the process of pairing with an Edge 520 and following a course - it's not exactly the same as the Edge 820, but similar.

You can't do this with your Fenix 5X, however.

You can use the Wahoo Fitness app on your phone (connected to the trainer by Bluetooth) if you want to control the trainer manually.