Assuming you have been volunteering intelligently form a few months since out last 'conversation', you are now going to take the skills that you have documented, and will transfer them onto your resume.
In the 'Experience' section, include the volunteering experience onto your resume the same as you would a regular job entry: your title, the school/organization you volunteered at, department (if applicable), timeframe, location, your duties. When describing what you have done keep the following in mind:
Quantify : how many students did you assist. You may approximate, average, estimate, etc.