There is always exceptions to the rule but the rule is never do your own cover. In fact the less input you have probably the better. I grew up with my mother as an artist and she is amazing. I learnt at her knee art but I knew I couldn't be a great artist. I understand all the theory but I don't have the passion to put in the hours of practice needed. When digital art opened up to me I could use the skills I had learned without having to be perfect at sketching.
You need those years of understanding and development of ascetics for you to make a good cover. To convince you why you shouldn't be doing your own covers I'm going to go over the basic skills an artist has that you might not have.
1. Colour Theory
Some colours go together some don't. Knowing which, is what artists learn at school and not just that yellow and purple are kinda cool next to each other but how the shades and hues differ in this as well.
Like Colour Theory there is an art to contrast. Clashing colours and images can work but mostly they don't. If you don't believe me just look at the fashion from the 60's and the 80's. Also, there is a time when contrast works and some times when you need to tone down that solid black so it blends better. Knowing when to use contrast and when not to is mostly trial and error and this frankly takes years to learn.
Unless you are doing a romance cover you have to be careful of cliches in the cover. I have rewritten this paragraph several times as I can't put into words the dreaded kitsch problem. These are over done and over used images. Some times you want some cliche as it helps state the genre but it has to be classy and the line is almost invisible.
This is my bug bear. When I first started I spent 6 hours looking for the right font for one of my first books. Now I'm a lot faster but unless you have spent hours and hours looking and studying fonts,don't choose the fonts. Fonts portray a message and unless you know what message you want you can't even start to figure out the font needed. Also, there are so many overused fonts. The only time you should state a font to your designer is if you are working on a series and you want the same font or you have chosen a font for your author name that is part of your brand.
5. Bad Blending
Combining two images is a finicky process and should take a while. The more practice the faster it takes but it generally takes me half an hour to blend two images together. Zoom is your friend but unless you are willing to be pedantic about it then you shouldn't be doing it.
6. The rule of thirds and the Fibonacci spiral
These are the two main compositional structures artists use. The first is about dividing the canvas in three and never having anything completely in the center. The Spiral is useful to direct the eye of the viewer around the cover.
Less is more when it comes to a cover. When a cover designer sits down they want to get across an impression. They do this with colour, font, lighting and usually one image. Putting all the elements in your story on the page is the equivalent of telling rather than showing.
8. Industry standards and Jargon
Every cover I do is 300 Dpi and 6 by 9 inches with a bleed. I tried my hand at audio books and it was this stuff, jargon, that completely tripped me up. You can learn it but my point here is whether you spend the hours and years learning or hire someone who already has done the long yards. Dpi is dots per inch, it is about how clear your image is. 6 by 9 inches is the size and is mainly for ebooks. This is a bit off for a print cover, depending on what size you want to go so you should have a different ebook cover to your print. Bleed is the bit around the edge that will be cut off when they print. A bleed allows the image to go right up to the edge but you have to also make sure none of the words end up in the bleed or too close.
I do think everyone should know these things here before they order a cover. It would make the designer's life so much easier. If you are going custom then tell them the concept you want to get across and the eye and hair colour of the main character and then leave them to it. You wouldn't go into a five star restaurant and insist on critiquing the chef's work as he is making your meal.
But if something feels wrong then say something. Artists are used to stepping back and critiquing their own work so when someone else does it, it really doesn't matter. I usually have others look at the covers I make. They usually articulate something that was niggling at the back of my mind. Below I have a list of things that are really easy to change and things that aren't.
Easy to change
1. Font style
2. Anything to do with the text
3. Colour of something
4. The size or position of something (though not always)
5. Transparency of stuff
Difficult to change
1. Blemishes and taking out bits of people (yes, these can be changed but it takes a little longer)
2. Effects (many effects require several steps and when you want a change all the steps have to be redone unless they have created a file to automatically do the effects)
3. Adding anything (remember how long it takes me to blend images that is what adding anything is about)
4. Making things less blurry (it is easy to blur something but not the other way around)