I made the cover above in about an hour (maybe less). It's properly formatted and proportioned for a 6' x 9' CreateSpace cover (for a roughly 250 page book).
Before you start making your own cover, make sure you know the dimensions that your printing services require. The cover above is entirely make believe, but I made it as an example of what your end product should look like (if you don't mind a minimalist cover).
I don't have Photoshop so I use Pixlr but you could also use something like Gimp or SumoPaint if you prefer. While SumoPaint offers more features than Pixlr, I can never seem to get it to work for me without crashing mid-project.
I put the CreateSpace template as the background image and built a solid black Layer 1 over it (my true background). Make sure that your background is a separate layer from the template so that you can refer back to it throughout the process.
You'll probably want to start with adding an image (Layer 2) and blending it or erasing around the edges with a large, soft circular brush. I was going for a minimalist approach so I only added an image to the front but if you prefer, you can find one that wraps all the way around. You can pay modest fees and royalties for photos or (if you're thrifty and patient) you can search through Free Stock Images (or browse Deviant Art and ask the artists' permission for commercial use. Please be responsible and make sure they have proper release forms from anyone featured in the image). Or ... if you or a friend have a high end camera, feel free to use your own images. They'll be original and you won't need to worry about copyright issues (or duplicate covers).
Another quick note about the image and placement that I used - I purposely chose this image because I felt that it would draw attention to our "Dramatic Title". Studies have proven time and time again that when you see somebody, it's a natural reaction to follow their line of sight. Thus, if you choose an image with a person's face (or at least their eyes) I would recommend that they are either facing in a direction that interacts with the overall design (in this case looking up at the title) or an image that looks directly at the viewer (forcing them to make eye contact with the product). One of the only exceptions I can think of for this would be perhaps where you have multiple people in the image (hopefully interacting with one another). This is more characteristic of books in the Romance and Erotica genres.
For Layer 3 I added a soft charcoal gradient that fades in diagonally from the upper right hand corner towards the man's face. It might be difficult to see on the image above, but at full size the effect is more apparent. Take note of the perspective and direction of the lights and keep it consistent throughout the cover. Adding a soft texture (in this case it probably wouldn't be visible) or a soft gradient is a tiny, super easy step that tricks the eye into seeing it as a single image, rather than a face randomly planted on top of a black background.
Click on Layer 1 and adjust the transparency until you can see your template underneath. Place a white shape over the ISBN box. (This is Layer 4.) It doesn't need to end up on the end product but this will keep you from entering that space with the rest of the text on the back.
If you have an "About the Author" section with a high quality, professional looking photo of yourself or a very small emblem to place on the spine, go ahead and add them now. (Potentially Layer 5 - 6)
If you're using Pixlr, you'll probably find that the font sizes are adequate for use on the back of the cover, but they're nowhere near large enough for the front. I believe the only text not added on Pixlr was the title and author name on both the front and the spine. Though I did it here, I would hesitate before recommending this since it's difficult to ensure that the same fonts will be available on different programs (and unless you're using black and white, finding the same color is nearly impossible as well).
Be pragmatic. Save it as a layered file then flatten your image (don't forget to readjust Layer 1's transparency) and save it as either JPEG or PNG [if you're still not sure which one is better for your project see here]. Like I said, I really don't like using Pixlr for text. You can try using ipiccy but my personal favorite is still Canva. You're limited in options but the accessibility of the site is outstanding. It might not work if you want more stylish or personalized text, but I always prefer to stick with more standard text anyway (though they do have quite a bit to choose from).
I'd love to see the covers you come up with. Let me know how everything works for you below.