The first thing that you will need to do is to have the image in front of you, so before you do anything else make sure that you have the opened the image.
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After you have it open create an adjustment layer but do not make any adjustments, only click O.K. When you have done this then choose the layer adjustment mode and then multiply. This will cause your image to get much darker as its density becomes greater. If you feel that this is not enough then you can always redo this again until you are satisfied.
If however you find it too dark then you can make it lighter by sliding your opacity bar to the left until you are happy. When you have achieved the effects that you require you can now repaint the second adjustment layer parts that you wish to alter with your paintbrush.