See the ScalaDoc for PageRenderlet and its superclass AbstractRenderlet for usage instrcutions. Note that the resulting rendelerlets need to be registered as service, in serviceComponents.xml the respective section would look like this:
<scr:component enabled="true" name="org.example.html.BookFormRenderlet">
	<implementation class="org.example.html.BookFormRenderlet"/>
	<service servicefactory="false">
		<provide interface="org.apache.clerezza.platform.typerendering.Renderlet"/>
	</service>
	<property name="service.pid" value="org.example.html.BookFormRenderlet"/>
	<reference name="renderletManager"
	interface="org.apache.clerezza.platform.typerendering.RenderletManager"
	cardinality="1..1" policy="static" bind="bindRenderletManager" unbind="unbindRenderletManager"/>
</scr:component>