With NPrinting, you would simply schedule a job containing your report task to run once a month. No variables required.
Since you are new to NPrinting I suggest you go through our Quick Start Guide. It contains links to resources that will get you started: http://www.vizubi.com/nprinting/quick-start/
Here is a tutorial that shows how to schedule jobs: http://community.vizubi.com/knowledgebase/articles/261185-how-to-schedule-jobs
Hope this helps.