well, living in the sunny south i've never done one, but it should be straightforward.
1. locate the old heater. hopefully its on the passenger side of the block, mounted into one of the 1-5/8 freeze plugs. or it might be in a radiator hose...
-- see if you can just get a new gasket for the old heater -- might save a few bucks.
2. determine the replacement. buy it and get it in your hands. also buy 2 gallons of new antifreeze concentrate and 1-2 gallons of distilled water.
3. drain the radiator and block when cold or cool. radiator cap off. radiator drain is odd - drivers side, PLASTIC, turn about 1/4 turn, then pull out, then turn another 1/4 turn and it'll come all the way out. dispose of old antifreeze so you don't kill the dogs and cats. if block doesn't drain well, or you can't get the radiator drain open, then just disconnect lower radiator hose.
4. remove and replace block heater. tighten to spec's and don't damage or overtighten any rubber seals.
5. refill radiator with your appropriate mix of antifreeze. since you're needing a block heater, you likely need a cold mix. pour in antifreeze about 1 quart at a time, then some distilled water. then another quart, then water. etc.
fill overflow tank to max with 100% antifreeze.
6. park with front end up on an incline and leave radiator cap off. start engine and allow to come up to temp. check for leaks. top of radiator. monitor overflow tank for 1-2 weeks as the system purges any remaining air. if you hear gurgling in the passenger side dash, its remaining air in the heater core. it'll usually work itself out.
here's an example of a block heater in freeze plug hole.
it just uses an o-ring as a seal - so MAYBE if you have one like this, all you need is an o-ring from the local parts store, hardware store, or plumbing supply.