If you want to modify the existing CHDK sources to build your own version, you need: * Knowledge of the programming language C. (online tutorials and resources) * A simple text editor to modify the source code or an advanced texteditor with syntax highlighting (for example Notepad++) or a C development environment. * The source code (How to get it). * Instructions how to compile the CHDK, which can be found here: Linux, Windows and Mac OS X. * A SVN-client may be useful if you want to participate in the CHDK development or if you want to keep your own version always up-to-date (see here).