This is easily one of the questions I get most often and it is actually easy to answer. Most authors would say, "Don't. Have a professional designer do it.". I personally love this answer, but will be fair here. Can you design your own cover? Sure, but keep it simple. This is what I mean.
The best advice I can give is to actually DO the design yourself. Do not hire friends or family. One of the toughest things is to tell a loved one that you are not happy with their work. The only thing harder than working with those you are close with is working for yourself. Yes, I just said you could but, I never said it would be easy. We are our own worst critics. Any of the design work I do to promote my own business is difficult. It is much easier to help others with design.
Step one. Get good software and familiarize yourself with it. Play around with photography. Try different effects and save often.
Step two. Keep it simple. Try to stick with one photograph or main element. Do not create a collage of images. For the most part this technique is for family portraits and does not make a good cover. Remember your design will be viewed as a thumbnail.
Step three. Thumbnail means tiny. Do not use small text unless you are not concerned with it being readable. Do not use small design elements. No image in a mirror, seen from a window of a house in the distance. That is called a "dot". Your main focus of your design should be LARGE and everything else should be designed around that main idea.
Step four. Do not use cheesy cars sales ad design effects. Unless you are designing a car sales cover. Keep shadows and glows minimal. Avoid florescent or bright colors (they do not print). Do not outline the text as it renders poorly in small sized on screen. Don't shout your design.
Step five. Perhaps the most important one. Do not get obsessed with the small details. You will never find the exact right image for your design. This is one of the biggest benefits to hiring a professional designer. We can look at your thoughts and concepts objectively. Design requires some flexibility. Whether that be due to the skill of the designer, available stock or complexity of the design concept.
Step six. Feeling overwhelmed? Spending hours looking for stock? Can't seem to recreate that perfect effect you had right before your program quit and you lost your work? Contact a designer. I can give you simple advise or point you in the right direction. My design work is more affordable than you may think. Time is money, after all.