The demand curve is a function typically seen on graphing paper. If you know several sets of prices you sell an object for matched with the quantity demanded at that price, then you can create your demand curve. Then, you can see how much quantity will be demanded at any price by drawing a straight line from the price you want to sell the object until it meets the curve. The spot it meets the curve is the quantity demanded.
Label the Y-Axis "Price" and the X-Axis "Quantity Demanded."
Plot your given data of quantity demanded at a certain price. For example, if you have a price of $5 and a quantity demanded of 100, then mark a spot at $5 on the Y-Axis and 100 on the X-Axis. Repeat this for each data set.
Draw a line over the marked spots, fitting it as best you can. This is your demand curve.