I know this post is old, but the above question was left open, and precisely yesterday I had to go through this with the Ukatex covers we just received.
I only took pictures of the result (sorry about the low light), however I'll try to explain how I did it.
Front seats, I unscrewed two plastic Torx screws at the front of the seat base, then with my hands I worked out towards the back detaching the plastic, until it was only held by the recline knob and the lumbar knob. I don't know how to remove these. This, at least for me, was enough to fit the cover. With my hands, I pushed the end of the cover so the black elastic cord and the white elastic straps were reachable from the back. I passed the cord through the straps and made a knot. At the front there are two straps. I didn't know what to do with them, but I managed to hook one into a piece of metal of the seat base, so was in tension. The other one I left it alone, but when I pushed all the plastic around the seat back into place, the cover seemed to be pretty tight.
For the seat back, it all goes with velcro. At first I did not understand the bottom part, but then I worked out that you have to remove one side of the velcro, so it can be attached to the soft material that comes from the other end. Hope I'm explaining myself!
Armrests, with my hands I removed the plastic cup, so I could at least cover the top end of the cover.
Bench, The bench back it all goes with velcro too, so this is pretty obvious. With regards to the seat, I did move the bench forward, so I had access to the rear. I removed the case where the jack lift goes. Then, and I think this was very useful, I attached two strings to both ends of the black elastic cord. This allowed me to pass the cord through the various white straps, and then make a knot using both ends. Once the knot was done, I removed the helper strings. Finally, to give the straps more tension, I open the drawer under the seat, and used four of the hooks that come with the seat covers to tighten the black elastic cord. There are places where you can hook under there.
If something is not clear, let me know, and I can try to explain better, or take a picture.