Getting a book cover template is the best way to get started on your full cover. You can grab a template from KDP or Ingram.

KDP

The KDP template creator is for fast templates which only take into consideration the Trim Size (your book dimensions), Page Count, and Paper Type. These are needed for the obviously, the height and width of the book, but the page count and paper type will determine how thick the book is, and so those are needed to calculate the width of the book. The output of the template designer is a zip file with a .png file and a .pdf file. Either of these can be used for the layout. Here is a sample of the output:

The template file shows the edges of the trim, the bleed over regions, and the safe places for important cover matter. Just keep all important information inside the orangish-red sections. The yellow barcode region is where the barcode should go unless there is a specific reason why it should be elsewhere on the back cover. Just important this cover into GIMP and start designing your cover, using this overlay to test your positions. Remember your image should be created in 300 DPI resolution.

IngramSpark

The cover generator for IngramSpark is more robust, containing a lot more information about the template. The major advantage of the IngramSpark generator is that it also includes barcode generation, so you do not need to purchase a barcode when you buy your ISBN. Again, the regions are self-explanatory, but this template will have extra space for instructions. If you use this template on KDP, crop off the extra space and instructions, or they will become part of the cover!

Both tools are excellent, and I use them together to create my book cover design.