To create a new custom task you only need to go to the “Task Management” and press “create new Task manually” at bottom of the page.
The Workflow Designer will ask you some questions, how you would define the new task.

1. The key
This is the slug of the Block, which will be used to store the relation in the database. Must be unique and must not contain spaces!
2. The classname
Must be a value with the Prefix “WfTask”. For example WfTaskNewBlock. This is the classname of the new block.
3. The label
The label, visible in the sidebar and below every block in Designer. Should describe the function of this block.

Then the Workflow Designer will create the necessary files.
The block will be created in the task group “special tools”.

Now you need to modify the files.

WfTask*.php

You found in /modules/Workflow2/tasks/.

This file must have a special structure.

[prism field=sourcewftask language=php]

The require_once in second line will make sure all Workflow related functions are available.
This line is important, because later I will make sure you could call every type externally.

handleTask(&$context)

This function will be executed every time the task will be executed and must include all php code you want to run in this situation. (But you could also include external files like in every other php script)
This function MUST have a return value, which is equal to one of the output points you configure in task.xml or “yes” if created manually in Task Management.
The parameter is from the Type WorkflowVTEntity, which will be explained Here.

beforeGetTaskform($viewer)

This function will be called before the configuration popup is shown. You could prepare all dependencies and assign values to the $viewer parameter, which is the Smarty Object used for the popup.

beforeSave(&$values)

This function will be called before the configuration values are saved. The $values variable is an array with the submitted fields in the task variable.

WfTask*.js

You found in /modules/Workflow2/tasks/.

Will be loaded in the configuration popup and could handle all functions for this window.

WfTask*.tpl

vTigerCRM6: You found in /layouts/vlayout/modules/Settings/Workflow2/tasksforms/
vTigerCRM7: You found in /layouts/v7/modules/Settings/Workflow2/tasksforms/ 

The wildcard in filename will be replaced by the task key for this file and not the classname, like before.

The configuration template.
There exists an open <form Tag, which will store any value you write to the task[…] variable and give this value also back to the templates $task variable.