It's not that complicated at all. All I did was create a new Layoutpage in the Masterpage Gallery through the webinterface.
Since the normal page content types use the rich html field for the content I created a new content type derived from the page content type. But you can skip that step and just use a different field in the page content type which contains plain text.
Then I opened that newly created layout page in Sharepoint Designer and added the RadEditor control.
After you've created the layout page, you can create new pages on your webinterface and choose the new layout.
You don't have to modify anything at all.
All I added to the layout page was following:
<%@ Register TagPrefix="rad" Namespace="Telerik.SharePoint.FieldEditor" Assembly="RadEditorSharePoint, Version=220.127.116.11, Culture=neutral, PublicKeyToken=1f131a624888eeed" %>
<rad:RadHtmlField FieldName="plainTextField" runat="server" DisplayWidth="100%"></rad:RadHtmlField>
If that doesn't look familiar, you should reconsult the installation notes for the RadEditor.