You derive new simple types by restricting existing types.

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!