Trying to save myself some work

I’m busy with an assignment for college. I have to design and implement a simplified structure for a real estate company in XML and provide a DTD for it. I’ve written a small XML file with a few clients, employees and realties, with the employees being at the top of the hierarchy, encapsulating both clients and realties at the same level. I haven’t really thought this through, but it’s the only way it made sense to me at first glance so I went with it.
Now I’m too lazy to type the DTD from the ground up, so I’m looking for ways to extract at least a rudimentary one from the XML file. So far my search for a suitable tool has only brought up defective or somehow crippled software. Guess I’ll have to do it myself after all. Bummer.