How to track leg machine exercises during Strength training?

Use Lap button to switch between sets and rest, then update reps set count on post-set rep count screen as usual. Upon syncing to Connect, update exercises and weights in the exercise list.

Alternatively, if the workout you do is the same, you could create a custom strength workout. This way both the exercises and intended weights will be prefilled, and you only need to update rep counts.

I like the idea of creating a custom workout; one for leg press and one for leg curls  How do I set that up?

Training -> Workouts in either mobile or web Garmin Connect. 

Got it. Thanks.