Hi Chris,
For single leg cadence drills if you are completely new to these the struggle to start with is to fire up your neuromuscular system in order to perform the move properly.
If you need to, lower your resistance on the bike and get your cadence as close as you can to the recommendation. But again if you are a little slower, this will come with time and practice.
As for the ‘non working’ leg, the first time you can leave it clipped in but make sure you don’t put any resistance through it, if you do want to go unclipped, I recommend bending the knee so your leg is out of the way of the pedals but you are still maintaining a straight position on the bike.
Happy drilling