Should be pretty easy considering the age of the truck. If the radio is new, and universal, it should have come with a wiring diagram. If its out of another vehicle, you will need some info on what all the wires are for.
You'll need a place to tie in the positive (red) wire. Its best to tie it into somthing thats only hot when the key is on. Poke around the fuse panel with a DMM or a testlight and see what all you have to choose from. There is most likly a fuse set up for a radio, but if not, just make sure its tied into somthing that can handle the extra load. The ground (black) can be tied to somthing metal in the dash. Use a eylet and star washer to make sure the connection is good. Some radios also have a wire that gets tied into the headlights to dim the display when the lights are on. Some also have a wire that is hot all the time for memory and clock functions, if you have the wiring diagram it will show you what all the colors are for. If you don't have one, you will need to get one.
Does the truck have speakers in it? I think in 81' you could still get a single speaker radio. You may have to add some in the doors or somthing to get decent sound.