11. How do I write my own DTD?

You need to use the XML Declaration Syntax (very simple: declaration keywords begin with <! rather than just the open angle bracket, and the way the declarations are formed also differs slightly). Here's an example of a DTD for a shopping list, based on the fragment used earlier:

<!ELEMENT Shopping-List (Item)+>
<!ELEMENT Item (#PCDATA)>
	  

It says that there shall be an element called Shopping-List and that it shall contain elements called Item: there must be at least one Item (that's the plus sign) but there may be more than one. It also says that the Item element may contain only parsed character data (PCDATA, ie text: no further markup).

Because there is no other element which contains Shopping-List, that element is assumed to be the ‘root’ element, which encloses everything else in the document. You can now use it to create an XML file: give your editor the declarations:

 
<?xml version="1.0"?> 
<!DOCTYPE Shopping-List SYSTEM "shoplist.dtd"> 
	  

(assuming you put the DTD in that file). Now your editor will let you create files according to the pattern:

<Shopping-List>
  <Item>Chocolate</Item>
  <Item>Sugar</Item>
  <Item>Butter</Item>
</Shopping-List>
	  

It is possible to develop complex and powerful DTDs of great subtlety, but for any significant use you should learn more about document systems analysis and document type design. See for example Developing SGML DTDs: From Text to Model to Markup (Maler and el Andaloussi, 1995): this was written for SGML but perhaps 95% of it applies to XML as well, as XML is much simpler than full SGML—see the list of restrictions which shows what has been cut out.