Common instructions on handwashing are too vague for me.

Scrub your hands for at least 20 seconds.

Sure, but how? I'm left guessing at which parts to scrub how much, possibly ruining the effect of the washing.

If you want to be precise and thorough in your handwashing, some studies will give you more details. Shi, O'Donoghue, et al (2023) did a nice observational study on how washing technique affects cleaning results. The study uses the same seven steps of scrubbing as the Hong Kong Recommendation. Figure 2 shows the estimated washing effectiveness as a function of the duration of each step. If we take the peak of each such curve, we conclude that optimal handwashing goes like this:

1. Wet hands
2. Apply soap
3. Rub palm-to-palm, 4 seconds
4. Rub palm-to-dorsum, fingers interleaved, both ways, 6 seconds
5. Rub palm-to-palm, fingers interleaved, both ways, 2 seconds
6. Rub backs of fingers in palms, curled together, 4 seconds
7. Rub around both thumbs, 5 seconds
8. Rub fingertips in palms, both ways, 6 seconds
9. Rub around both wrists, 6 seconds
10. Rinse
11. Dry

The linked post includes a video to teach this procedure.