is it a poster that will fold into itself? Concertina? Something you make up yourself? Figure it out on scrap paper.
3 mm bleed
5 mm safety
add 1 mm for each fold mark.
add the amount of every fold onto the measurements, eg 5 folds, add 5 mm.
set the front and back up as separate files.
can be nice to put a little extra on the front so that when you fold it up it comes together without seeing the inside bits.
Crop marks - add when you save.
if you have thick card consider that you'll need extra room for the fold.
think about how it will look when it folds in on itself. DON'T WANT IT TO BUCKLE.