Testing and statistics. You want to make sure that the stats that are to be balanced can be accessible externally via a script language such as LUA or XML. Don't ever hardcode stats: because you want to exclude the programmers from the balancing problem and have them work on more important parts of the project. It is then…
You start by knowing how to script fluently, because otherwise you're boned. Firstly, examine what you've got and what can be salvaged. Do you have a baseline of assets you can't really afford (money or time) to replace?