Take some time to look through this sample piece of XML and see how it is constructed.
The code is shown here:
size="large" onAction="conBoldSub" imageMso="Bold" />
size="large" onAction="conItalicSub" imageMso="Italic" />
size="large" onAction="conUnderlineSub" imageMso="Underline" />
Chapter 11: Working with the Ribbon 135
Figure 11-4 Custom ribbon control Contoso