tinyMCE_config.js

// TinyMCE Settings.
tinyMCE.init({
	extended_valid_elements: 'style',
	mode: 'exact',
	elements: 'id_name',
	theme: 'advanced',
	theme_advanced_toolbar_location: 'top',
	theme_advanced_toolbar_align: 'left',
	plugins: 'safari, style, table, paste, media',
	theme_advanced_buttons1: 'bold, italic, underline, justifyleft, justifycenter, justifyright, justifyfull, formatselect, fontselect, fontsizeselect',
	theme_advanced_buttons2: 'pastetext, pasteword, search, replace, bullist, numlist, outdent, indent, blockquote, undo, redo, link, unlink, image, forecolor, backcolor, hr, charmap',
	theme_advanced_buttons3: 'visualaid, tablecontrols, |, code, removeformat, media',
	content_css: '/path/to/rich_text_editor.css',
	paste_retain_style_properties: 'background, border, color, font-family, font-size, font-weight, line-height, text-align, text-decoration, text-indent',
	auto_focus: 'id_name',
	width: '500px',
	height: '300px'
});

// If using Prototype...
document.observe('dom:loaded', function() {
	tinyMCE.getInstanceById('id_name').execCommand('mceSetContent', false, $('id_name').value);
});

// If using jQuery...
$(document).ready(function() {
	tinyMCE.getInstanceById('id_name').execCommand('mceSetContent', false, $('#id_name')[0].value);
});