In example extension (ezcomment) with new action which create new comment directory structure looks like:

extension/ezcomments
 |
 | - actions
 |        | - content_actionhandler.php
 | - settings
 |        | - content.ini.append.php

You will have to override content.ini in your extension and add
extension/mynewextension/settings/content.ini.append.php

[ActionSettings]
ExtensionDirectories[]=mynewextension

For ezcomments extension this will be extension/ezcomments/settings/content.ini.append.php with code:

 
<?php /*
 
[ActionSettings]
ExtensionDirectories[]=ezcomments
 
*/ ?>

Body of new custom action must be placed in:
extension/mynewextension/actions/content_actionhandler.php

This is general structure of new Action:

 
<?php
function mynewextension_ContentActionHandler( &$module, &$http, &$objectID ) 
{ 
   if( $http->hasPostVariable("CustomAction") ) 
   { 
       //your custom action goes here 
   } 
} 
?>

Function name have to start with extension name, other ways it will not work

In ezcomments example code will look more complex, since it will create new comment object.

code from extension/ezcomments/actions/content_actionhandler.php

 
<?php
include_once( 'lib/ezutils/classes/ezoperationhandler.php' );
 
function eZComments_ContentActionHandler( &$module, &$http, &$objectID )
{
	if( $http->hasPostVariable( 'AddComment' ) )
	{
                        // fetch object and read node ID
		$object =& eZContentObject::fetch( $objectID );
		$nodeID = $object->attribute( 'main_node_id' );
		
                        // read user variable
		$subject = $http->postVariable( 'Subject' );
		$author = $http->postVariable( 'Author' );
		$message = $http->postVariable( 'Message' );
		
                        // prepare new object data
		$parentNodeID = $nodeID;
		$userID = 10;
		$class = eZContentClass::fetchByIdentifier( 'comment' );
		$parentContentObjectTreeNode = eZContentObjectTreeNode::fetch( $parentNodeID );
		$parentContentObject = $parentContentObjectTreeNode->attribute( 'object' );
		$sectionID = $parentContentObject->attribute( 'section_id' );
    	
		$contentObject =& $class->instantiate( $userID, $sectionID );
		$contentObjectID = $contentObject->attribute( 'id' );
 
		$nodeAssignment =& eZNodeAssignment::create( array( 'contentobject_id' => $contentObject->attribute( 'id' ),
		'contentobject_version' => $contentObject->attribute( 'current_version' ),
		'parent_node' => $parentContentObjectTreeNode->attribute( 'node_id' ),
		'is_main' => 1 ) );
		$nodeAssignment->store();
 
		$contentObjectAttributes =& $contentObject->contentObjectAttributes();
 
		$loopLenght = count( $contentObjectAttributes );
 
                        // fill up content object attributes with user data
		for( $i = 0; $i < $loopLenght; $i++ )
		{
			switch( $contentObjectAttributes[$i]->attribute( 'contentclass_attribute_identifier' ) )
			{
				case 'subject':
					$contentObjectAttributes[$i]->setAttribute( 'data_text', $subject );
					$contentObjectAttributes[$i]->store();
					break;
				case 'author':
					$contentObjectAttributes[$i]->setAttribute( 'data_text', $author );
					$contentObjectAttributes[$i]->store();
					break;
				case 'message':
					$contentObjectAttributes[$i]->setAttribute( 'data_text', $message );
					$contentObjectAttributes[$i]->store();
					break;
			}
		}
 
		$contentObject->setAttribute( 'status', EZ_VERSION_STATUS_DRAFT );
		$contentObject->store();
 
  		// publish new comment
		$operationResult = eZOperationHandler::execute( 'content', 'publish', array( 'object_id' => $contentObjectID, 'version' => 1 ) );
 
                        // redirect to weblog full view		
		$module->redirectTo( '/content/view/full/' . $nodeID );
	}
}
?>

The simplest form for new custom action will look like:

<form method="post" action={"content/action"|ezurl}>
   <input type="submit" name="CustomAction" value="Custom action" /> 
   <input name="ContentObjectID" type="hidden" value="102" /> 
</form> 

ContentObjectID have to be included in POST.

In ezcomments extension example from will include Subject, Author and Message input fields. Code blow can be placed in weblog_full.tpl. User will see form which can use for posting new comment:

<form method="post" action={"content/action"|ezurl}> 
	Subject: <input type="text" name="Subject" value="" /><br />
	Author: <input type="text" name="Author" value="" /><br />
	Message: <textarea name="Message"></textarea><br />
            <input type="submit" name="AddComment" value="Submit" /> 
            <input name="ContentObjectID" type="hidden" value="{$node.object.id}" /> 
</form> 

That's all. We have new action that create new comment under weblog object. Code from content_actionhandler.php (ezcomments) need of course some additional checking for incoming data, validation, etc. but shows how custom action works in the eZ publish.