The first thing you need to do is see if you can pair your trainer to your Garmin. I don't know how Garmin head units work because I use Wahoo and Stages....but once the trainer is paired....you just start pedaling (with the Wahoo). It will automatically display your options for indoor riding (ERG, Sim, etc...). With the Wahoo, you just load a course and start riding. The courses are automatically downloaded to the Wahoo from RWGPS.

With the Stages (I'm going to assume this is more Garmin-esque), you create a separate profile. In this case it will be "indoor"....within that profile you have several settings you can assign (such as continuous recording with GPS off, Power - choose trainer or power meter, candence - same, Movement - Trainer, etc..)….then it's just a matter of selecting "sim" mode and loading a route you want to ride.

Sorry, I'm not more help....but dig into that Garmin and see if you can pair it and create an indoor profile.....or pair it and load a route....start riding and see what happens.