Time is money.
If you are interested in building one from scratch, then go to the AB website and search for their DF1 protocol document.
If you consider your time to have value, consider purchasing RSTrendX, an ActiveX program that be dropped into any container (Word, Excel, VB) and will do the trending for you.
RSTrendX is included, free, with RSView, RSLogix (although perhaps not the starter version), and perhaps others. You can probably get the program for free (though not especially legally), but I believe that it requrires a higher version of RSLinx Lite (which IS free) to work.