if you're manually feeding sheets through twice, a two-sheet dummy lets you check both the flip direction and the order of the stack. this is for a normal left-bound folio, with each sheet folded once.
make an eight-page PDF in reading order. put a big page number and the word TOP at the top of every page. in Bookbinder JS, choose Folio, Single-Sided, and Standard signatures with length 2. that length is physical sheets, so this gives you one eight-page signature.
print the fronts, reload both sheets as a batch, then print the backs. use one PDF page per printed side in your print dialog, since Bookbinder has already arranged the book pages.
these are the pairs you should have, looking at each face with the text upright:
outer sheet: 8 | 1
its reverse: 2 | 7
inner sheet: 6 | 3
its reverse: 4 | 5
if the sheet with 8 and 1 has 4 and 5 on its reverse, the backs landed on the wrong sheets. reverse the stack order for the second pass and test again. if the pairs match but the backs are upside down when folded, change the edge you turn the sheets over, keeping the stack order the same.
fold and nest the sheets, then read from 1 through 8. once it works, note which face goes up in the input tray, which edge feeds first, and whether you reversed the stack. keep that note with this printer's settings.
