Open a new document in Photoshop with a size of 350 x 300 pixels. Add a new layer by clicking on the Create a New Layer icon in the layers palette. With this layer active (a in front of it in the layers palette), add a Layer Mask by clicking on the Add a Layer Mask icon in the layers palette. This will make the layer mask automatically active. We're going to use this mask in step #3 to add to make