That all depends on what rules you are trying to add, and if its simple, class mechanic changes or something more crazy.
For house rules that use the editor like modifying how classes work, creating variation on how feats/abilities or spells work use the following method:
- Create a new file with the modifications you want.
- Make sure EVERYTHING has the a new source that use
- Source Name: The name of your House Rules (whatever you like)
- Source Unique Id: a short code that normally begins with a capital letter and can be no longer than 8 characters (i think 8)
- Parent Source: House (parent source: "House" indicates that is in the parent source section for house rules. Very Important.
- Save and test file.
This should get you started, and also assumes some knowledge of how to make the rules you want. That's beyond the scope of this question I think. Once you have done this you can select or deselect whatever house rules you wish to use. Keep in mind that not all house rules can be made in this way, some require XML coding which, I suggest you don't mess with, or ask someone else to help with (not me, I wouldn't touch that XML with a 40-ft pole.)