How complicated is it to install the thermocouple that comes with a portable digital controller?

The easiest way to install it is through a drilled peephole plug. Or you could drill a thermocouple hole in the wall of the kiln. If you are firing two kilns, one at a time, with the same controller, you will need to transfer the thermocouple from one kiln to the other. Insert the thermocouple into the kiln wall until it extends into the firing chamber by the amount shown in the controller instructions. This is usually around 3/4". Then place a mark on the outside of the thermocouple that is even with the kiln case. During firing, check the thermocouple occasionally to make sure the mark on the thermocouple is still lined up with the kiln wall. That is your assurance that the thermocouple tip is extending the correct distance into the firing chamber. Mount the thermocouple to the kiln so that it cannot fall out during firing. If you drill a hole in the kiln wall and insert the thermocouple, don't let anyone tamper with it. If the thermocouple falls out or is pulled halfway out, the kiln can overfire.