Excel charting - Use Trendline (6 mos Rolling Average)

I created a Line Graph for OTD% and need to add a 6 month Rolling Average with Trendlines. I'm not sure how to use trendlines in Excel graphing, can some one assist me or send me to a helpful link? This is the first step...then I will need to find out how to get it to display only the current month and past 5 months.

Thank you.