There are many threads that explain how to use the mod selector.
U open the bat file which opens the mod selector. the swf file is in the folder. select the mods and press "build" when everything went fine u should have another swf file in that folder now. this one u have to change with yogda to make it a 12.3 like explained in the other thread.
Please take a look at the other threads when there are any errors while building or changing version.