From a local Git clone of http://git.eclipse.org/gitroot/sirius/org.eclipse.sirius.git, and provided you have Maven 3 installed, you can build Sirius locally from the top-level directory with the following command:
mvn -f packaging/org.eclipse.sirius.parent/pom.xml clean package
The resulting update-site is then available in
To install the result in an Eclipse instance, you will also need to add the Nebula repository at http://download.eclipse.org/technology/nebula/snapshot/.
The documentation is available in the help system under the "Sirius" top-level category. You can also browse the doc from http://www.eclipse.org/sirius/doc.