Paint your walls in a neutral color using a crackle paint medium for effect. Follow the package's instructions for applying the medium. If your walls are old, peeling, smudged or paneled, leave them -- it helps add to that rustic cottage look.
Decorate the room with old, mismatched furniture pieces. It's okay if they're worn or tattered in places.
Decorate your fireplace. If you don't have an actual fireplace, consider getting a fake one. Place a large iron kettle or cauldron in it where you can burn incense to make it look like a smoking brew. Place candles or lanterns on the mantle, and lean and old-fashioned broom on the side of it.
Hang framed images of pressed herbs, printed pages that look torn out of an old spell book or images of odd-looking creatures and creepy people. Use old, antique-looking frames you find at garage sales and second-hand outlets.
Decorate shelves with a collection of mismatched old jars and bottles. You can put herbs, colored water or other strange looking items inside the jars.
Place charm items around the room. This might be a voodoo doll with pins in it on top of your sewing basket or little pouches hanging above the doors. If you find an interesting plaque or necklace with odd-looking symbols, hang them on a wall hook or over the edge of a chair to look like the room is being magically protected with amulets and talismans.
Collect some very big books and stack them as a side table. Add a few more books throughout the room. If you are not satisfied with the book cover or jacket, take it off, or make another cover out of butcher paper.