It is not difficult. If all you want is simply a drop shadow, it's a lot faster and easier to simply use one of the preset, canned versions. But you can do a lot more with one you make yourself.
In this instance I put my image onto a blank file. I inserted a blank layer between the two layers - the blank, white layer on the bottom and the image layer on the top. Put your cursor on the thumbnail
of the top layer and Ctrl-click
. That puts marching ants around the entire image; it has selected the image.
Now, with the blank layer active: Edit > Fill Selection. Before you do this, make your foreground color black, or whatever color you want your shadow to be. You are not going to see anything because your black box is exactly the same size as the image on top which is covering it up.
Now use the move tool to slide that box out from under the image 'till you see what you want -- I moved it down and to the right. Apply a gaussian blur and reduce the opacity of that layer 'till it "looks right".
Now, you ask yourself, "Why bother, it's so easy just to use the drop shadow effect?" You are right, if all you want is a simple drop shadow. But, because it is on a layer, and is not an effect, you can do all sorts of things to it. A drop shadow will exactly follow the shape of what it is applied to. A "layer shadow" however, can be transformed and pulled into just about any shape you want. This technique is commonly used in "Border Breakouts". Go to the Challenge
section of the forum and look at this week's Crop Challenge #3
. Find the entry I submitted and you will see an example of a transformed shadow.
Give it a try and have fun. Ask Qs if you get hung up on anything.