I rode for 15 years without ever managing to idle, then I saw someone doing it and learned very quickly. Moral: meet other unicyclists and you’ll learn more quickly.
First thing: imagine that your uni has only one pedal. That’s the working pedal, and you can eventually learn to idle one footed. When you idle 2 footed, you should be concentrating virtually all of your effort on the one pedal.
So, taking your left as the working pedal (you may prefer to use the right - it’s a personal thing)…
The working pedal passes from just below horizontal, through the bottom of its travel and up to just below horizontal. On a clock face, it would move from about 8 through 6 to about 4. (Ideally, it should be from 7 through 6 to 5. The better you get, the smaller the movement.)
Now, as you can already juggle, I guess you do various balancing tricks. Imagine balancing a broom on your finger. Now set the broom swaying. The way to keep it balanced is to move your finger from side to side, so that the SUPPORT (your finger) passes under the centre of mass (say the broom head). The broom head only moves a little bit; your finger moves quite a long way. The better you are, the less your finger has to move. The more you want to ‘perform’, the more you will move your finger.
So with idling the uni, the SUPPORT (the point where the wheel contacts the ground) moves quite a long way (say 1/4 - 1/3 of a revolution) but the centre of mass (your bum) hardly moves at all. Idling is like a pendulum movement, with the wheel being the bob, and your centre of mass being the hinge at the top.
So that’s the movement you’re after.
Now concentrate: Weight firmly in the saddle. Plenty of pressure on the bottom pedal. Light pressure on the top pedal. Look at something a long way away. Keep the movement smooth. Pass from wheel in front and about to fall back, to wheel behind and about to fall forwards.
Now how to learn? Some people recommend holding onto something. Some others recommend the following, which is what worked for me:
Ride forwards slowly, and stop with your ‘working pedal’ down. then ride on.
When you can do that, ride forwards and stop. The ‘working pedal’ will rise slightly behind you because of your momentum. Push it down so that you do a half (or third) revolution backwards. Regain control and ride off.
Practise this [stop, back, ride off] several times.
Now try [stop, back, forward, back, ride off]
When you’ve got this far, it’s simply a matter of practise practise practise. Count pedal strokes and aim to beat your best score. All the stuff you did when you were first learning to juggle.
You may well find that learning to idle and learning to ride backwards come more or less together. The skills are related.
For performance purposes and general improvement of skill, you may want to practice the super idle and mega idle.
In a super idle, the working pedal is allowed to pass over the top, so that the wheel does 1 1/3 revolutions instead of 1/3 revolution. In a mega idle, it’s 2 1/3 revs each way.
Other things which will help are finding a smooth floor, and pumping your tyre up very hard. Get your seat as high as you comfortably can. Later, when you are confident, you may want to lower the seat a bit and soften the tyre.