I chose this picture of Britney Spears from "In Style Magazine." Now, you should have something that looks like this

Next, click file, new. Make sure the Width is set at 1 pixel, and the height is set at 2 pixels. Name It "Vertical, then click OK." If you are having trouble, look at this picture.

Now, click the zoom tool, pictured here

Click on the small box that you just made until it is maximized all the way like this

Next step is to take the paint brush tool like this

Now paint only the top pixel by clicking on the top part several times until completley black. It should look like this(*NOTE* you must make your brush size 1 pixel. The setting is located at the top left of photoshop. You must also make sure your mode is in Normal mode.)

Next, go to Edit, Define Pattern, and press OK. It should look something like this

Now, click the Paint Bucket Tool. Make sure it is NOT the Gradient Tool. It should look something like this (*Note* make sure that the "Fill" is set on "Pattern" which is located on the top left part of Photoshop.:

Now, click on the tool-bar part of your picture once. Following this, you will want to create a new layer. You can do so, by going up to Layer, New, Layer. It looks like this

Then you will recieve a pop-up that looks like this. Copy the settings that are shown here

Now the new layer should automatically be selected. Now, just click once in your new layer with the pattern you have created, and it should look something like this

Now, play around with the opacity of the layer you have just created, look here for an example.

And WA-LA, you have just made a pattern. This is what the picture I made turned out to look like this: