I posted some links before on some older posts to youtube videos. Here is the check list you need to get to making some quality covers.
1. Get to know your software whether it be Photoshop or GIMP.
2. Find a good source for fonts for either retail or custom covers. I sometimes use
www.abstractfonts.com. They usually have what you're looking for...usually.
3. Get familiar with Clone Stamp, Healing Brush and their settings.
4. Get familiar with Color Correcting (Hue, saturation, brightness/contrast, levels, temperature and vibrance to name a few)
5. Filters. Blur filters and sharpening filters (always use unsharp mask in photoshop is what i was taught) and play around with settings.
6. When resizing keep the aspect ratio as close as original as possible to not warp the source image.
7. Cutting and slice tools. Use them and take your time. Don't rush the process it takes some time.
8. Patience.
9. Look for feedback and tips. Try asking specific questions as to where you are having challenges. The harsher the critic the better.
10. Youtube photo retouching and fixing videos because essentially its the same thing.
That should be a good starting point.