"Object Making" is the term used for altering the look of objects. "Hacking" is the term used for altering the object's function.
What Do I Need To Do To Start Hacking Objects for The Sims?
1) First step in altering an object is to have one to work on! For this you will need an object cloning program.
With this program you "clone" your own copy of the objects, assigning them their own unique Object ID number, also referred to as an 'GUID number'. It is this number and not the file name that defines your object as a new object instead of a replacement object.
PC users use Transmogrifier (T-mog or Tmog, for short).
http://www.lushcreations.com/
http://groups.yahoo.com/group/transmogrifiers/
Mac users use IffSnooper
http://www.atelierquebec.ca/iffsnooper/
2) Whether you are a Mac or PC user, be sure to pick up (or make) a "Magic Cookie" number before your start.
PC users, this is a number you will enter into Tmog (only once and it remembers the number for next time you want to make objects) and needs to be a number that no one else uses. T-mog will use this number to figure out what Object ID numbers to give all of your objects (and those must all be unique to prevent Object ID conflicts)
Mac users can also enter their magic cookie number just once. Yersinia, a budding mac sims creator and experienced story teller, posted instructions for doing so at the Simblesse Oblige Forums.
To enter your magic cookie number in IFF Snooper so that cloning or subcloning will automatically put your cookie in the GUID, go to the Edit menu in IFF Snooper and choose Enter Cookie (or, Command-K). You will then get a Magic Cookie box. Type in your original six-digit cookie number in the "Enter Magic Cookie" box, then hit Enter. Both your hex and two bites versions of your cookie will appear. Click OK, and that's it.
3) To Hack objects you need an iff editing program.
PC users have a few choices but the most common (and most complete) one is IffPencil2. (the "2" is now sometimes dropped so it's just called IffPencil and occasionally IP2 or IFF2). IffPencil 2 consists of the program (an exe file) and a text file (script.txt). They must be kept in the same folder otherwise IffPencil wont know how to translate all of those numbers into words.? IffPencil was never actually completed so does have a couple of little quirks but on the whole you can do pretty miraculous things with it. The author of this program has moved on so and it's now distributed "As-Is". IffPencil2 may be downloaded from The Sims Resource's Programs section.
Mac users will be delighted to find that their cloning program (IffSnooper) is also an iff editing program. The author is still actively developing this program. Recent updates to it have included a BHAV editor and BCON editor (BHAVs are where all of the objects coding is and are the core of object hacking. The BCONs house numbers that the BHAVs use.) Awesome stuff!
4) Learning to use iff editing programs.
Hacking tutorials were always a lot more scattered than object making tutorials. Most of the information was found in a few key sites and forums. As hackers moved on to non-sims activities and to The Sims2 these forums and sites dwindled away and domains were not renewed.
One that remains is Sims College. No longer given in the form of classes, this site has left it's material up as online tutorials for anyone who wishes to come learn it on their own. Unfortunately many people are having problems accessing the classroom now but the author of those lessons gave SSoW permission to host the lessons.
http://www.woobsha.com/SimsCollegeArchive/index.html
The PC program IffPencil is used for these lessons so Mac users will have to translate the material into IffSnooperese. As most tutorials available are written for IffPencil this is something you will need to do a lot and the creator of IffSnooper has written a tutorial to help with this:
http://www.atelierquebec.ca/iffsnooper/BHAV/bhav1.html
The two programs, Iffpencil and Iffsnooper, use very different layouts so it can be a bit difficult to translate between the two at first. A majority of the hacking tutorials cater to IffPencil making the already difficult task of learning to hack even more difficult for Mac users, but if you take it slow and be patient it WILL all click into place.