You can create the ribbon XML with any text editor. The Custom UI Editor is a bit more comfortable and performs XML syntax checking.
If you want to create the XML file with IntelliSense support you require either Visual Studio or e. g. the free Visual Basic 2005 Express and the CustomUI scheme.
Download and setup Visual Basic 2005 Express Edition.
Download the XML Scheme and unzip to a folder of your choice.
Create an empty XML file (create empty text document and rename .txt to .xml). In VB 2005 Express go to menu "File" / "Open File" and select your XML file aus.
If you don't see the "Properties" window, select "View" / "Properties Window"
In "Properties" click on [...] in "Schemes", then button "Add" and select the downloaded scheme ("customUI.xsd"). Conform the "XML Schemes" dialog with "OK".
Go to the XML file and enter < which will call IntelliSense.
(to enlarge click here)