I've done this plenty of times, twice in the last month and IM gonna do it at least once in the next few weeks. This is usually what I do, I use two felpro gaskets so it seals better w/o over-tighting the housing...doing so will warp the mating surface of the housing:
1-take off accessory belt to get it out the way and have more space.
2-drain a bit of coolant from the bottom of the car by turning the radiator draining valve
3- take off upper rad hose (thats why I drained some coolant, to avoid having it splash all over while removing hose)
4-remove two screws holding the t-stat housing together
5-remove the small hose attaching the housing to the block
6-remove electrical connector to coolant temp sensor
7-pull the housing out
8-clean surfaces on housing and block (i use gasket remover and a piece of sand paper)
9-using the first gasket and some blue RTV seal the t-stat to the housing and let dry for a few mins
10-put the second gasket on top of the first with some RTV so you now have the t-stat positioned on the housing and its being held by the gaskets...so you can now place it on the block w/o having the t-stat giggle all over the place and risk not seating correctly.
11-tighten bolts...DO NOT OVER-TIGHTEN
12-put everything back together & clean up excess RTV with a screw driver or other straight-edge. I usually let the silicone dry overnight to assure that it seals correctly.
Dont use a t-stat under 180*'s as it will not allow the comp. to run on a closed loop, besides it usually makes no notisable difference if you go cooler....Im using a 160 on my 94 and thats why im changing it..agian... plus imma change it on my 89 after doing the HG'S. I will be using 180* robert shaw t-stats which , for what i've heard, seem to flow much better than your average t-stats.