Here's the step-by-step. Let me know if you have any questions.
1. Get bench seats that have headrests from a wrecker, craigslist, etc.. My favourite is Dodge Caravan / Plymouth Voyager / Chrysler T&C 3rd generation ('96 - 2000). This is because they're nice and thick with neutral fabric upholstery. Plus they're easy to find so getting a matching set of 3 or 4 benches is fairly easy. And best of all, the sleeves are welded flat onto the top bar rather than being welded into indents in the top bar of the seat frame. This makes cutting them off easier. Only get the back benches if you can. The front benches & captain's chairs are welded in 2 spots on each sleeve so it's twice the work to get them off.
2. Strip the covers off both seats and mark where you want the headrest sleeves. If you have a plastic "seam" thing on your cover like the ones GM uses, do it at room temperature. I cracked one of mine trying to pop it apart in the cold. You need a flat-head screwdriver to get those seams apart.
3. Measure the top part of the plastic guide, #2 in the pic. Add about ⅛” for foam compression. This is how far the top of the metal sleeve needs to be below the top of the seatback foam.
4. Find someone who knows how to weld.
5. Make sure the welder understands what you're trying to accomplish and give him/her the headrest & plastic guides along with the stripped bench seats so that they can get the angle and width of the metal sleeves perfect. Basically all they need to do is copy the weld from the Caravan bench to your bench.
6. Weld the sleeves in place. Test out the guides & headrests before putting the seat back together.
7. Put all the foam back in place & cut or drill out the path for the headrest guide. You'll need to cut out a path down below the guide as well for where the headrest posts will go when the headrest is all the way down.
8. Come up with a way to keep the upholstery from unraveling where you cut through. I've been putting the covers on first and then slathering craft / fabric glue on the upholstery in a circle about 1/2" wider than the hole I'm going to cut. It dries clear so no big deal if a tiny bit shows. I'd rather have a 1/4" of crusty fabric under my headrest than unraveling upholstery
9. Put the upholstery on (if you haven't already) and carefully feel for the tops of the metal guides. Poke a hole with a metal skewer and feel around inside to make sure you know where the centre of the hole should be. Then cut out a hole large enough for the plastic guide. Make sure the hole isn't too small because if it is, there will be stress on the upholstery and might cause it to pull apart. I use curved nail scissors to cut the circular holes.
10. Put the plastic sleeves in place and put your new headrests on.
Where it says "top bar", that means the top bar of the seatback frame.
Here's a closeup of the metal sleeve + plastic guide setup. It's really very simple, and is just welded on to the front of the seat frame.
This picture was taken before I cut the foam to fit around the sleeve: