Since this is just to teach you how to make cloud, create a 300x300 pixels image. Of course, it doesn't have to be this size; you can choose to create a cloud anywhere you see fit, but for simplicity's sake, we'll use 300x300 pixels =). If you don't know how to, click on File > New and fill in the measurements. Now, create a new layer (Layer > New > New Layer, or Shift+Ctrl+N) and fill it in with the Paint Bucket tool. Choose your primary color as black.
Step 2
Next, select the Elliptical Marquee Tool. To get to that, left click once on your Marquee Tool, the first icon on your toolbar, and hover over the Elliptical Marquee. You'll be needing this to draw the cloud with :).
Step 3
Now, create a new layer. Since you have the Elliptical Tool selected, try clicking on the image and dragging across. An ellipse appeared, right? Okay, now comes the fun part of making it. Hold onto shift, and drag more circles near the first one. What does this do? When you hold shift, you notice the little plus sign near your mouse now, right? That means it's adding to the selection. On the bottom is the one that I did. You don't have to make one that looks exactly like it. Just add as many circles as you want and make a cloud of your own. Do as you see fit.
Step 4
Okay. Anyways. Next, select the Paint Bucket Tool again, choose the color white, and fill in the selection. You should see something like the one below.
Step 5
And now, here's your final step! On your Layers pallete, you should see something called Opacity. Don't know where that is? Click here. What is opacity? It controls the layer's transparency. So the lower you change it to, the more transparent the layer becomes. Change the 100% to something lower. I'll choose 60%, this way when you are making your cloud above another image, you'll see the background image as well as the cloud. I ended up with this.
Generally, you can add these clouds to anything. See, here's something that I used clouds for:
