Boris' tutorial can be found
here
OmegaRed :
If you want to play by your modded rules permanently, without ever playing with the default rules again, you can edit Civ3mod.bic (or civ3x.bix if you own PtW). But be aware that modding can have unexpected results and bugs, so I suggest that you save the original civ3mod.bic/civ3x.bix in a safe place beforehand.
If you want to play by your modded rules from time to time without overriding the default rules, simply launch the editor, and do like MattH says.
After you have edited your rules, save it under any name (like
mymod.bic). To play with your modded rules, select in the Civ3 menu "Start a Scenario" and choose your mod in the list.