This sort of depends on your source.... If you are using FSoD there probably isn't a source file you will have to decompile one of the SWFs using RABCDASM or whatever it's called (drawing a blank here) but essentially you are looking for a folder that has the most folders within it as anything else in your source... that is probably the client files. Then you open up that FOLDER using IntelliJ. However, make sure you are using the PREMIUM version of Intellij (there is a free 30 day trial and you don't have to put in any credit card information or anything). You will also want to download the correct SDK... pretty sure this depends on the source as well, someone else will be able to tell you better probably. Basically you have to make all your changes and then compile it using the sdk and boom you have a baby SWF