Hello there. First, you have to place a chest and then on top of the chest you have to put a sign. On the sign write this: [Trade] (Item you're selling) (Item you're buying) Your username If you want you can also place it on a block right behind the chest.
Actually, you don't have to fill in your username. If the rest is done correctly, it fills in automatically. The sign can be placed on the chest by Holding shift and click, or you can place it on a face of a block directly above the chest. Also, you might want to put the amount of each item before the Name. If it doesn't accept the Name or if it's too Long, use the item ID instead. Keep in mind you cannot buy / sell more than a stack of each item (p.e. 64 cobble, 16 enderpearls, 1 sword)
