Here's An Easy One

Witch's Curtain
Lay a heavy-duty black garbage bag flat on a work surface, open end toward you. With scissors, cut 1 1/2-inch vertical strips from the bottom, stopping a few inches from the top. Cut open one side seam, then unfold bag. Use masking tape or tacks to hang. Then, with hands a few inches apart, grasp each strand and pull; stretch strands in several places for a decrepit effect.

Such a cute idea, especially if you have a front porch, as I do....hmmm...

More Martha Ideas:
Scary Message Lanterns
Tombstones
Witch Silhouette
Scarecrow
Glitter Pumpkins
'Lumina' Pumpkins