There are two ways to apply Clipping Mask, You will use both of them. For this purpose you are having already arranged layers. In clipping mask the bottom-most layers shape works as Mask for the image on the layer above. So make sure that in the layer palettes, the base layer is directly under the layer that you want to be clipped. In this demonstration you will see leaf shape as base layer. For more information please look at Photoshop Clipping Mask Day-8 Tutorial Video in CC.