I have this 20+ year old mugo. I want to get it out of the landscape and make it a bonsai. Any advise on getting it out?

Is it growing out of a rock? How big is it?

Looks like you have no access to the rootball. To get it out, you need to be able to get to the roots. If the roots are under that concrete slab.. Walk away. (Or remove the concrete)

I would hesitate to dig this up. Wrong time of year right now, and mugo do not like root disturbance. Damaging the roots in removing will be a killer, I agree with Leatherback, leave it where it is. Find a smaller one in a pot and grow that on.

It is in a planting garden- the roots my be deep but I should be able to get to the root ball. I was thinking of doing it in dead of winter. When the tree is dormant. Any thoughts on how much of the root ball could be removed ? Any experience trying to layer and old trunk?

You will not layer it, as I said mugo do not like roots disturbed at any time. and you will have o cut a lot off to get into a container of any size. I would not attempt digging it up as the likelihood of survival is slim. If you must try then you will need as much of the root ball as possible,