They are varible in difficulty, mine was done with the throttle body still installed.
If your car is compliant you drain some coolant or clamp off the coolant hose to the thermo element and remove the hose from the thermo element. Then using a phillips head screw driver remove the screw from the c-shaped retainer on the front of the throttle body and once the screw is removed, remove the retainer. The thermo element on mine slid out of my throttle body without too much effort using a pair of channel locks. The new unit will come with a spring loaded carrier, you will need to apply pressure to the spring to remove the new element. Clean the opening of the hole, then put a little coolant on the oring of the new unit to lube it for easier installation. I was able to put the new one in by hand. After the new unit is seated, replace the retainer and install the screw. Put the hose back on and top off or refill coolant.
Be careful when applying pressure while removing the element as sometimes heavly corroded throttle bodies can break a chunk out and then you have to replace the throttle body. If yours is a PITA, then you will need to remove the throttle body and do it on a bench. Hope this helps you.