It's not too difficult, just a bit time consuming. Let me preface this with the disclaimer that there would be more than one way to do this. I figured this out as I went along and haven't had any issues since. However, if you see a better way, feel free to use it and let others know.
For the bottom cushion(s), you can either leave the old ones on, or remove them for a slightly tighter fit. I opted to leave them on for ease of assembly. These were also assembled in the car without removing the seats. The P5 seats have a 60/40 split in them which allows access to the fuel pump. The new MSP cover can be added in on top of the old cover by "folding" the seats about half way up and stretching the cover across them. The seats are then to be treated as one big "seat" even though they will still be two separate cushions. I utilized self adhesive velcro strip down the middle to secure the seats from shifting. Underneath at the back, I used a combination of safety pins, velcro and elastic thread to keep things sturdy (not a huge issue, the top part of the seat largely keeps things in place). Finally, the front of the seat fabric has a metal bar incorporated into the fabric. I used the same combo of safety pins, elastic thread, and velcro here as well secured around that metal bar.
The tops of the seats required some more involved work. The original seat covers need to be removed in order to install everything cleanly. The difference between the P5 and the MSP seats (aside from the obvious) is the incorporation of the headrest. I took some medium density foam block purchased from a fabric store and "sculpted" the shape of the MSP headrests (a rounded triangle). This was then placed into the seat covers before placing them on the actual seats. I also removed all of the plastic seat release knob components and cut a slit into the MSP seat covers for reinstall at the correct locations.
Next, the seat covers were physically installed on to the P5 seat foam/frame in the same manner as the old ones. Once completed, I used some felt backing sourced from the local fabric store to custom cut a new backing for the seats in the style of the originals. This was done because the MSP seats have several holes in the backing for fold down release access. These were then installed onto the seat backs. The backing would be optional, as it can only be seen when the hatch is open and the cargo cover lifted. After this, the seats were reinstalled into the car and all of the miscellaneous tie downs and seat release knobs were reinstalled.
Please let me know if you have any questions on any of this. Unfortunately this was a project I did without taking any pictures or notes. If I was thinking, I would've documented things for a better how-to.
