The basic UCG process is really very simple.

Identify a suitable un-mined coal seam and:

Sink two wells into the coal seam: one through which to inject oxidants (for partial combustion), the other, some distance away, through which to remove the syngas to the surface
Heat the coal at the base of the first well
Inject and regulate a flow of O2/air and, sometimes, steam (to ensure the gas separates, through partial combustion, into syngas, as opposed to burns)
Remove syngas to surface

http://www.hardassetsinvestor.com/features/3161-underground-coal-gasification-an-old-energy-revolution-whose-time-has-come.html?showall=&start=1