Run the following in the project directory:
./build.py build
This will create a build with a name in the form adblockplus-1.2.3.nnnn.xpi. This file will contain the source code currently in the repository and all available locales.
To simplify the process of testing your changes you can install Extension Auto-Installer. Assuming that Extension Auto-Installer is configured to use port 8888 (the default value), you can push your changes to the browser by running:
./build.py autoinstall 8888
The extension will be updated immediately.
To verify your changes you can use the existing unit test suite. The unit tests are a separate extension that is installed in addition to Adblock Plus. You can either install the existing unit test builds or clone the repository and create your own build. After installing the unit tests go to chrome://adblockplustests/content/index.html to run the tests.