How do I sum the total of a particular field to insert in my footer at the end of my database report?

How do I sum the total of a particular field to insert in my footer at the end of my database report? I have one field that I would like to sum at the end of my Database report and insert into the page footer. It is just simply trying to find the right function to insert into the footer. Probably would like to have a Report Footer to insert the summed data in so it won’t show on each page footer.

See my answer in this post - click here. Has step-by-step & sample if needed.