[Q] [TWRP] How to create a script for using sdext on every ROM? topic

Hi XDA users!

How can a script for using the sdext partition of the SDCARD is done?
I am interested to build one for the Sensation because of its little data partition but I don't know how to do :(
For example for this ROM: http://forum.xda-developers.com/htc-...anich-t2911152

Every details or ideas are welcome :)

Many thanks in advance!