I was just browsing this forum and came across this thread. I decided to join up to reply to this thread as I went through the same thing with one of my past P-cars (on my 3rd now, another Boxster). There wasn't a whole lot of info on this process then either, and some of the Renovo reviews had me apprehensive to try it.
I did this on my 08 911 C2S Cab (now sold). It had a gray top that showed dirt and creases really bad. It was also prone to getting grease stains from the retracting top mechanism. I dyed it black after reading of another 911 owner who reportedly did the same, but he didn't have a lot of detailed info on the process.
Prior to applying, I washed the top throughly with soap and water and then the canvas cleaner to get out all dirt and contaminants, then I let it dry out thoroughly. Dye won't adhere well to dirt.
I bought a big roll of plastic that folds out like a painting tarp (looked like something Dexter would use), a large and small nylon brush, a sponge, painters tape, 1 Liter black Renovo Reviver kit that included a bottle of cleaner and a black dye. I also bought a 1 Liter bottle of the Renovo proofer that repels water. I only used around a half a liter, but I figured it wouldn't hurt to have extra in case I needed to do touch ups later.
The Reviver is thin like water colored paint and tends to run, so I covered everything with plastic / newspaper. I put the Dye in a plastic bowl and applied with the nylon brushes and sponge in a shaded garage. I read in reviews it's important to apply in the shade. If you do it outside it can dry to fast and give an uneven result. My dad is a retired art teacher with plenty of painting experience so I had him lend a hand. He worked on one side while I worked on the other. After applying one full coat until top was thoroughly wet and saturated evenly, I let it dry over night with a fan on it. The next morning we applied a second coat then let it dry until that evening. Finally, we applied a couple coats of the proofer which is sort of like a water proofing agent that causes water to bead off. I suspect this also may help keep the color from washing out. Also, on the 911, I had to dye a section of fabric covering the retracting roll hoops behind the rear seats. The Boxster top should be easier as there is significantly less material.
It did not completely hide every little scuff or crinkle and look brand new like I hoped, but it did look like any other regularly used and maintained black 997 top afterwards and the coloring was even and consistent. It was easier to keep looking nice than the gray top was and fabric blemishes were less noticeable. I was happy with the end result and it never washed out or faded at all over the next year I had the car. I regularly went through high power touchless car washes too. The proofer did a really good job of beading water, so I will likely add this to my current Boxster's pristine roof to help protect it. I don't have a ton of photos of the finished result as I usually took photos with top down, but here is one I found. It was a dreary fall day, but hopefully you get the idea.
Just a disclaimer, I found some scary reviews of Renovo where people tried to use it for a color change on a tan or blue roof and they were very unhappy with the results. Some ended up with a color banding effect and complained of it drying to fast or washing out in some places, but I suspect they were likely trying to apply it in the sun to a top that wasn't thoroughly cleaned or sealed afterwards with proofer. My dad has been painting for over 40 years so he is very familiar with painting techniques and proper prep work. Just as you can get awful results painting a room, you can get great results with the same paint if you know what you are doing. The thing I've learned from my Dad is that most of the time is spent prepping (cleaning / masking) and waiting (between coats). The actual time spent applying paint (or dye or proofer in this case) isn't very long. It is not a very challenging DIY as long as you plan it out, and it helps if you have a second person to lend a hand. Good luck, should you decide to try it.