The basic structure is:

<xs:simpleType name="sometype">
   <xs:restriction base="parent">
   ...
   </xs:restriction>
</xs:simpleType>

The 'base' attribute points to the type you are restricting.

All values must be valid values in the base types' value space.

Inside the 'restriction' element you place your restrictions!

All restrictions are elements with a 'value' attribute.