Extensions are packaged as the Java EE web applications and packaged as the normal .war files. To deploy the custom extension, you may do as for any other web apps.

In Tomcat, this ends up by copying the war archive to the webapps folder.

However, the GateIn extension mechanism imposes that the starter.war webapp starts after all extension wars.

This is the case for the sample applications bundled by default.

There are several ways to control the loading order of webapps in Tomcat. Please refer to Tomcat's Deployer How To.