Step 4 - Duplicate It
Click on the layer of the shape. Which would be the thing were making. Now press ctrl+alt + the up arrow. Once you do that that will duplicate it but also move it one pixel up. Repeat the proccess but this time use the down arrow. Go to layer/layer style/ and fill it white. So now you should have the white shape on top. Grap the rectengular marquee tool again and cut it like it shows in the image. Once you have that press backspace to erase it. Then go to layer/layer style/blending options and make the opacity to 25 also make sure you take off the stroke. This is what you should have so far.