Hi,
As far as I know, there is no manual as is. There are web pages (thecloudstrap, baselines inc, SmartDXL...) and some videos in youtube that will help you. But the most important is the DXL Reference pdf in https://www.ibm.com/docs/SSYQBZ_9.6.1/com.ibm.doors.requirements.doc/topics/dxl_reference_manual.pdf
The problem I see in your case is how to practice if you have not a data base to test. Talk to your Doors admin, maybe he/she can create a database for that purposes, even give some advices or documents.
Good luck!!
Jorge