You make a stencil out of a something you have around the house (cardboard, pizza box, brown paper bag, foam core) in the shape you so desire. Large letters or arrows pointing to the party, possibly even a turkey or pumpkins and squash would all be good ideas.
Next you place the stencil on the grass, mist the area with water and then sift flour on. If you want colored designs and have a spice grinder, just grind your flour with some food coloring first. You can check out the results over at Pinterest and Pink and Green Mama.
(Image: Brooks McElwain)