To make a miniature BILLY bookshelf:
a) either accept defaults for sizes in mm for things like height, width, depth, and board (plank) thickness, or change the values - the defaults give you a standard 6-shelf Ikea billy bookcase
b) set shelves variable to change the number of shelves
(for example, to make a smaller bookcase like the Ikea 3-shelf unit, cut the value of the height variable in half (from 2020 to 1010) and change the shelves variable from 6 to 3
c) set skirting and back variables to true or false (decide whether to notch the back of the shelf base or whether to include the back of the bookshelf)
d) set scalefactor variable to the amount by which the bookshelf will be downscaled
(for example: to make a 1/24 scale bookshelf, set scalefactor to 24)
Then compile and export to STL for printing.