in openscad all what you have to do is, changing the values of the named variables
on the top of the code, (all variables are commented)
you have 4 sections where you have to change what you need:
/ [Box dimensions] / <-- as indicated you put the box dimension there
/ [Box options] / <-- there you setup some options
/ [PCB_Feet] / <-- if in option you set the PCBFeet value to "1"
/ [STL element to export] / <-- there you choose which part of the box you want to export (do it part by part)
in OpenScad, press "F5" to preview what you changed
then press "F6" to view what you will export,
then press the "STL export" button to save your STL file that you will print.
Feel free to check the animations I published to see how it works.
Have fun ;)