How hard is it to install a car stereo?

My neighbor gave me their old car. It's a 1991 Ford Escort. I'm thinking about putting in a new stereo How difficult would that be cause I've never done it before.

Not difficult at all if you do research beforehand

Ask sonneone thats done it to help you with it

25 years old makes it a classic and an antique. Forget the stereo. Restore it to it's original condition.
Make sure you have title from the previous owner, so he doest want it back.

Leave your car alone. Your new stereo will just get stolen after a week.

It's going to be hard for someone with no experience with stereo and limited experience with cars, electronics and tools. It's worth having a professional installer do it right the first time.

Look up the car's color code and match it to the stereo's color code. If you don't understand that, get someone to help you. The new stereo needs to be connected to power, ground, panel light/memory, antenna and all the speakers. It will only sound as good as the speakers are able to play. That car has OK speakers unless they are worn out, but aftermarket improvements are available. The job is as easy as doing a small jigsaw puzzle, but you will have instructions with a new stereo.

HU's in old cars are simple. You'll need to find a wire harness and a dash kit -- COULD be a little tricky with a car that old, but they were extremely common so I would expect you'll be able to find what you need.
You pull a panel or two off the dash, take out probably 4 bolts holding the factory unit, disconnect a couple wire bundles.
Install the new dash kit which will hold the new radio in place. Connect the wiring harness to the stock wires and to the new HU, bolt it up and reinstall the dash pieces.

It all depends on the "person's experience" with whatever that person is working with. It will be hard for a novice like you. It's very easy for an experienced car audio enthusiast.