Creating a Joomla SOAP Plugin
To create a Joomla SOAP plugin you need to create a Joomla plugin of type SOAP. In the plugin you create your functions and you register the functions you want to be available in the webservice. You can download an example plugin, which also creates a demo webservice in the component so you can do some tests. Download the Hello World SOAP plugin here: http://www.joomla-specialist.net/extensions/examples/plg_soap_helloworld.zip
Complex Types Support
You can also add your own complex types to your webservice within the same plugin. Download the Hello Complex World SOAP plugin here: http://www.joomla-specialist.net/extensions/examples/plg_soap_hellocomplexworld.zip
Creating a webservice
In the component you can create a new webservice in the Webservices section. For each webservice you can specify how you want the webservice to behave. For instance, you can specify which of the installed and enabled SOAP plugins will make up your webservice. Thus, the registered functions from the selected plugins, will be the functions available in your webservice. If you don't select specific plugins, all the registered functions will be part of the webservice.
You can also specify an authentication method. You can manually enter a username and password, or select an existing Joomla user or even an access level.
Note: This is still in beta and if you find bugs or have a great idea please submit them at the website: http://www.joomla-specialist.net/forum/general/feedback.html