First, a story I wrote about my journey. Not very prescriptive, but might be a bit inspirational:
http://bikrutz.org/triblog/?p=3516
Next, a link to the first program I followed, which opened several doors to the process I followed to success: https://melissahadleybarrett.com/penile-rehabilitation-program/

And, the elements (without the evidence-based justification behind them) of my rehab program. Not saying it's the only, or even the best way to proceed, only that it's what I did, and I succeeded:

1. I bought and followed the program Melissa Bradley Barrett offers in the link above.
2. I bought a vacuum erection device - https://vacurect.com - and used it ~ 5 times a week, specifically, pumping up for three minutes, releasing and milking out the engorged blood from my penis for 2 minutes, pumping again two more times. Total of 15 minutes.
3. I took the prescribed sildenafil (generic Viagra) 50-100 mg about 4-5 x a week.
4. Bought a vibrator designed for a flaccid penis and used it to induce orgasm 3-4 times a week. (https://www.hotoctopuss.com/products/male-sex-toys-award-winning-and-best-selling-pulse-solo-essential-dragon-eye-vibrating-penis-masturbator-special-edition)
[#2-4 intended to keep the nerves involved in orgasm and blood flow in the penis functional while the erection nerves recovered. There is some scientific justification for each of them.]
5. During the whole process, my wife was a patient and supportive partner. While playing around in bed after the first two months was frustrating, it kept us connected as a team trying to return to whatever level of intimacy we could create at the time. Without that, I'm sure success would not have occurred.
6. I realised I was rebuilding my sexual function from scratch, and worked on each of the elements separately before trying to put them all together: read the last half of my story above for more insight.

I think it helped that my post-op PSA was "undetectable" 6 weeks, 3 and 6 months post-op. This freed up emotional bandwidth for the penile rehab as I wasn't worrying about my cancer...