Add “wysiwyg” editor in magento Custom Module

Go to the file {your magento root folder}/Block/Adminhtml/{module name}/Edit/Tab/Form.php add following code in top of the file

$fieldset->addField(
	'followupemail_text',
	'editor', 
	 array('name'=>'followupemail_text',
	 'label' => Mage::helper('followupemail')->__('Content'),
	 'title' => Mage::helper('followupemail')->__('Content'),
	 'required' => true,
	 'style' => 'height:24em;','config' => $wysiwygConfig, 
	));

go to the file {your magento root folder}/Block/Adminhtml/{module name}/Edit.php add the following line in this file

protected function _prepareLayout()
{
   // Load Wysiwyg on demand and Prepare layout
   if (Mage::getSingleton('cms/wysiwyg_config')->isEnabled() && ($block = $this->getLayout()->getBlock('head'))) {
      $block->setCanLoadTinyMce(true);
   }
   parent::_prepareLayout();
}

Note : Never use “content” as field name.