A few years ago, my wife and I bought and installed a black leather seat cover kit for our 2005 xB. Installation was not difficult, but did take patience. Total time for us was about 14 hours, including removing and installing the seat assemblies. In my opinion, this is something an expert should be able to do in less than four hours.
The front seat cloth is not glued on, and pulls off like removing a pair of tight gloves. Remove the headrests and do not take out the plastic bases in the top of the seat until the cloth cover is removed. Take out the plastic drop-down hooks from the back of the front seat. If your leather cover is not already cut out for these hooks, and you never use them, you may not want to put them back in.
There are about 100 hog rings to be removed for the total job. Use a pair of regular pliers to twist and remove the old rings; or they can be cut. For closing the new hog rings, a pair of hog ring pliers is about $7. We used medium sized hog rings for the install.
The front seat bottom and front seat back are set up similar. There are thin metal rods which are used to keep the covers tucked in tight against the foam padding. These also have hog rings and as you pull the cover off partially, the first rod is exposed and removed, then there is another one a few inches further. The seat bottom cover is pressed onto the front of the seat frame with a long, narrow u-shaped clip.
The top seat cover pulls off in the same manner. There are two or three of the tension rods in it. After the cloth is off, remove the headrest's plastic base. Reach under the foam padding and use a wide pair of pliers to squeeze the tabs together and then slide each tube out of the top. Some new seat covers may already have holes cut for the headrest posts; ours did not. If necessary to cut, be careful to have the new covers positioned exactly where you want them. Otherwise the opening in the leather will show around the plastic base.
For us, putting the front seat top cover on was the most difficult part of the job. We tried stuffing the foam padding into the cover and aligning it without much success. Finally, we turned the cover inside out and then pulled it down over the seat like a sock. Each tension rod had to be installed in sequence as the cover went on. Be sure to have the cover positioned were wanted before installing the final hog rings.
The front seat bottom is installed by putting the narrow clip on in the front and working the foam padding into the cover. This is the second hardest part of the job. Here again, the tension rods are installed as the cover is fitted.
What makes this difficult for the both the top and bottom sections is the foam padding expands slightly when you take the cloth off. Then you are compressing the foam to put the leather on.
The rear seat cloth is glued on; but is easy to remove. You can chase the cloth off with a razor blade and keep the cover intact. This took us a long time, but it made for a smooth surface of the foam padding. I pulled a couple of foam chunks out, but after the leather was on, where those small pieces were pulled out did not show through.
I have read some postings where people just cut the sides off of the back cloth and left the center parts. For us, the two top sections and the bottom of the back seats were a lot easier than the front seats. Make sure the top seat covers are just where you want them before cutting the opening for the three rear headrests.
We removed the cloth from the two front headrests and from the two larger back headrests, but the center backseat headrest fit perfectly into the leather cover with the cloth left on. This will all depend how your leather headrest covers fit. On all of them we left the original piping; I could not figure how to remove that without damaging the foam padding.
There is carpeting on the back of the rear seat top section which is held on by plastic tabs. There were seat belt slots already in the bottom section of the leather covers.
Finally, the cloth inserts on the door panels. Our leather kit came with the four leather panel insets. This was something we could not install ourselves, no matter how we tried. We took them to a professional for installation.
Good luck with however you decide; the leather interior is beautiful. Please post pictures if you undertake the project.