How do you decorate a 12 foot long mantle?

You are so lucky! Here are some sites for inspiration.

http://www.midwestliving.com/homes/seasonal-decorating/holiday-ideas/christmas-mantel-decorating-ideas

https://www.countryliving.com/home-design/decorating-ideas/g1571/holiday-mantels/

https://www.pinterest.com/patinaparadise/christmas-mantels/

Candles picture frames holiday decor for the holiday coming up a big mirror .

Lenora, Decorate it with end to end garland. Next, use some cinnamon scented pine cones, spread out, maybe in groups of three,. Some large Christmas balls, tinsel, and some Christmas picks which you can find at the Dollar Tree. When the holiday is over, you can decorate it with knick knacks, some books, candles, figurens. Aloha!

Given the height of the stonework, make use of vertical space with taller decorations or plinths to raise items on the mantle tovary heights.

A woodland display of florist's greenery with deer, twigs spray painted white to mimic snow covered trees, and twinkle lights would enhance the setting.