Replacing your late model healiner is really easy once you get the old one out. Here is what I did on a 1989 Ford Ranger XLT. I first took it out and laid it on a table/stand that's handy.
Just nice and slowly peel off that saggy hanging mess and toss it in the trash. Next you will need to remove all of the foam backing from the very delicate fiberglass backing. This looks harder than it is and repairs are simple, see below. That's right! Time consuming but it works excellent. Take a low to medium speed drill (electric or air) and a multi-wired semi soft wire brush, not the large heavy gauge wire wheel that's made for high speed wire wheeling.
Check it out! Sometimes it's a little harsh if you're not paying attention, but never mind those small bare patches, you'll never even see them once you're done.
Below you'll see a big ugly hole with a finger... Well just tape it up from the back like the following picture.
Tape is good.
Then you have to stabilize the fibers so that your new headliner has something to stick to. I used an entire can of Black Semi Flat Rust Tough Enamel paint by Krylon. It's good and strong and penetrated the fiberglass backing quite a way's in making an excellent new backing.
That black surface allowed to dry about 3 hours. I goofed up and didn't take pics of spraying the headliner adhesive but here's what you do:
1) Lay the new headliner (got mine on E-bay from auto krafters) and center it so you have the entire pad covered.
2) Peel back one half slightly farther than half way.
3) You can get the Headliner glue in a 15 ounce spray can from your local parts store. They come with 3 settings in the spray nozzle, light, medium and heavy, I used medium.
4) Spray the exposed backing, and the headliner and allow 4 or 5 minutes to dry, spray evenly and try not to miss any thing by crossing your pattern on the backing.
5) Once several minutes have passed, slowly and evenly lay down the headliner with the smooth even pressure from the PALM of your hand, from the Center out to the edges.
6) Then turn it around and repeat for the other half. Below are what your results will resemble.
The new headliner is basically ready to handle one to two minutes after gluing the two halves together. Trim up the excess and you're ready to go.
Badda Bing Badda Bang, You're Finished.