Started by having a single nail that I wouldn't bite - all the others were fair game. You start becoming quite obsessed with that one, noticing how much better it looks than the others etc, and being proud of how that one is unbitten.
I did the opposite -- I chose a sacrificial nail, and that one took the brunt. Once my other nails had recovered and I saw the difference, the habit basically disappeared on its own.
That nail has never completely recovered; it still looks stubby compared to the others even though I no longer bite it.
Started by having a single nail that I wouldn't bite - all the others were fair game. You start becoming quite obsessed with that one, noticing how much better it looks than the others etc, and being proud of how that one is unbitten.
Then simply extend that to 2 nails etc