The technique varies wildly based on what you actually want to hack... But the summarized version? You need to learn BHAV modding. There are tutorials around for that including a very good one by (I think) Carrigon.

To hack gameplay, you need to build a package with the BHAVs from in game that you want to change, then modify the contents and put in your downloads directory. The new versions of the BHAVs will override the defaults. To hack objects, you need to clone the object, then edit the BHAVs that are attached to it.

If you're going to try it, be ready for a lot of booting and rebooting your game, plus a whole heap of game crashes. ;) BHAV hacking is not something you'll want to do as your first sim project, work your way up to it: master recolouring, then master remeshing, then read up on SimAntics (and possibly also the .package file format), then try BHAV modding. Then you should be able to make your own hacks. :)

You need to master finding links to tutorials. Check the tutorial forum for links or the FAQ page. If you really couldn't find those, you couldn't have been trying too hard.

:)
Deedee