Minecraft wiki entry for Furniture, associated text: Take two of any block you want (you must be able to put an item frame on it!), place one of the blocks on the floor, and then place the other one on top of the block. Add an item frame with a clock on the top block and you're done! Something optional you could do is to add an item frame on the bottom block with a spade/shovel in it, rotated to be the pendulum.