Find the support and resistance levels. As above when you want to buy an instrument, its best to buy near support levels. The support is usually a previous reaction low. Using the same logic, the best place to sell an instrument would be near its resistance levels. The resistance level is usually a previous peak. After a resistance peak has been broken, it will usually provide support on subsequent pullbacks. In other words, the old high becomes the new low. In the same way, when a support level has been broken, it will usually produce selling on subsequent rallies - the old low can then become the new high.