Adding and removing source-files
After you added (or removed) a file to the codebase, the project files for MSVC, XCode and the CMakeLists.txt build files need to be updated.
Updating the MSVC / XCode project files
If you have Visual Studio (Windows) or XCode (OSX) in suitable versions for the project files in version control (currently git), add the files projects using the GUI, putting them under appropriate filters. There are approximately 10 projects, depending on the build and varying with time, including a common project for most game logic / mechanics, and the server and human and AI clients. Not all files need to go in all projects - UI code generally goes only the the human client, AI stuff only in the ai clients, common code in common - so only put files where needed.
Updating the CMake build files
In various subdirectories within the FreeOrion code structure, there are files named CMakeLists.txt. These are analagous to the projects in MSVC / XCode above, and list the source files to build for various components of the game. As with the GUI projects, new source files should be added to the appropriate lists in these files.
Updating Version Control
After adding source files, and adding them to the project files, you should add all relevant files to version control (currently git), and commit changes to any existing files.