Valid XML is actually easier to write than well formed XML because you simply follow the structure of the DTD, which requires much fewer choices. You must follow the DTD that you have written, making sure to not break any of the rules that you have written. If your document does become non-conformant, you had better fix it, as it will likely not display or process properly when you use it. Your document should include XML and document type declarations. Follow the DTD explicitly, paying special attention to the content rule of each element, making sure to comply where you are required to and choose where you are allowed.
Errors are a major problem for document developers.
If you take care in your writing, making sure to follow the DTD, you can avoid errors. Ensure that your elements nest properly, as overlapping elements cause errors. The DTD is yours, and can be modified as you need to. Keep the overall structure of your document in mind, being sure that your rendering of it in XML is correct and makes sense. Now that you are almost done the tutorial, you must be getting an idea of what XML is about.