On top of boxes B1-B3 it says:
"Rollover: Enter traditional IRA or pension distribution that was rolled over to a pension or **traditional IRA**. **Enter Roth IRA rollover or conversion on lines B5 or B6 below**" (my emphasis)
Moreover, after box B3 it says:
"Roth IRA Rollover or Roth IRA Conversion: Enter distribution, other than from a designated Roth, that was rolled or converted into a Roth IRA"
Box B4: "Amount of this distribution that may be rolled or converted to a Roth IRA". I did a very vanilla back door roth conversion (nothing in the traditional to start, $6k to the traditional, next day rolled it over to the roth, thus the 1099-R form), so in my case in this box I put the total that came out of the traditional IRA, $6k.
Box B5: "Check this box if the entire amount on line B4 above was converted to a Roth IRA". Yes in my case (and for many in this thread), so I checked that box.
Box B6: I left blank because I rolled the entire amount in B4.
The bottom line, again, is that 1) unchecking B1, 2) declaring $6k in B4 and 3) checking B5, in my case populated correctly lines 16-18 of form 8606. I had to fix this via forms this year because, in the Step-by-Step section, for some reason, I did not get detailed questions about whether this distribution from the traditional IRA was rolled over into a roth. And then, not surprisingly, form 8606 was missing data.
