This is a very,very easy job to do. Start by removing the heater hoses from under the hood (change to new is recommended, so slice the hose at the nipples for easy removal),remove any sealant or foam around the tubes and save aside for later, then from inside, remove glove box (simply release side retainer/stop and unhinge from main dash), next remove the h-core cover plate (a rectangular cover with about 4-6 screws visible thru glovebox hole). Now you will see the heater core first hand, which is easily pulled out of the cavity at this time, but, you will have to snake it out between the dash and the cavity after you pull it all the way out. No big deal to scratch up the old one doing this, but not good for the new one.At this time, clean the inside of the cavity with a good degreaser to remove coolant residue. Before you install the new one, you can gain more room by trimming back the 2 bottom locator tabs that help steady the h-core cover, they are about 3/4' long now, trim down to 3/8" or so, this will make snaking the new one in a lot easier.Once the new one is in, you should install the hoses at the core at this time, being pretty easy on the new core. If the core keeps sliding back under force, have a helper hold it steady from the inside while you push the hose further onto the nipples.When you have the hoses on and the core is as far forward as it can go, reinstall the inside cover and glove box, the tighten down the hose clamps. Before you start the engine, add any missing coolant to the radiator and overflow jug, and replace caps. Start the engine to check for leaks, if none seen , allow the engine to warm up 100% , then check for hot air available in all functions of the heater control. A note, I have had some trucks seep a little coolant out of the heater core cavity for a day or so after, don't worry , it is just old coolant that has accumalated in the box and should dry up after a day or so.Stop the truck and allow to cool off, then carefully remove the rad. cap to refill if necessary as well as the recovery jug.
Phil