There's plenty of write ups for doing this in 95+ where the seat bottoms are the same for XJs, ZJs, and WJs, but there's really no info out there (at least that I could find) for doing it with older XJs. Well now there is, for front seats anyway.
welder (you can drill holes and bolt everything together, but since I can weld I'm going that route)
13mm deep socket
Ok so find yourself a set of seats. Don't discount JY seats. I got mine for under $50 and they're in great shape. They're not heated, but you can add a heating element for ~$50. If you have to pull them yourself there's 4 13mm nuts holding the seats to the base in the 4 corners of the seat.
Pull your old seat. I'm starting with my passenger seat in case something goes wrong.
There's 4 13mm nuts hold the seat to the floor. You'll need a deep 13mm to get over them, but it's straightforward.
It's a lot easier to pull the seat if your doors are removable and you don't have to fight the door. Excuse the mess. Or don't, I couldn't care less.
Now you need to get the seat off the base. I don't have pics of this, but it's straightforward. Besides you're adults, figure it out for yourselves.
There's 4 13mm bolts on the bottom, remove them. Socket works fine on the rear 2, you'll need your 13mm wrench for the front two. Once they're off flip the seat over and the seat bottom will come off. After that you'll see 2 bolts and 2 nuts, both 13mm. Remove them and the top half of the seat will come off the base. Toss the seats, keep the base.
Now you're ready to modify the ZJ/WJ seats to fit the base.
You can see how much smaller the older XJ bases are than the new ones. The 4 corner holes in the old seat are where the later XJ/ZJ/WJ seats bolt to the base.
This is where I'm at, but I'll update sometime this week when I get the flat stock welded to the base.
The red lines I'm going to take the flat bar and place it on the inside of the seat over the holes and plug weld it to the seats through the existing holes. If you can't weld you just drill holes in the bar and bolt them in place.
The blue lines are going to line up with the base holes and then they'll be welded to the red bars. Again, if you can't weld you can just overlap, drill some holes, and bolt them to both the base and the red lines. I'm 6'3" so I'm welded them on the backside so I'm not losing ~1/2" of headroom.