Most print on demand companies will create and provide you with a custom size template for your book based on the interior copy you gave them.
This is a PDF template for one of my books that CreateSpace provided. It shows how large the front and back covers should be, along with dimensions for the spine.
Your eBook cover should be fine for the print book, it'll just need to be resized.
We showed you at the start of the book and website how the eBook cover for Self-Publishing was created by photographing some books on a bed sheet and then work with a paint and draw program to clean up the edges and make it look uniform. Then you simply add titles and author name.
For the back cover you can put a picture of you the author, so long as it's not a professional picture that you don't have the copyrights too and most professional pictures are like that! So do a selfie or have a friend take a picture of you! That and a bio will work nicely for the back cover.
And alternative would be a “blurb” about the book itself. Maybe with reviews if you got some good eBook reviews with 4 and 5 stars.
Those are your elements for the book. Most printers want the final mock-up in PDF format, but a few might actually want a JPG image instead. The process of doing this is the same, but the tools are different.
For a PDF film you will need to work in a word processor that exports a PDF file, such as Open Office or Word.
For a JPG image you need to work in a photo program like the free Gimp2 that lets you create a canvas using a scale of inches or millimeters.
The rest of the process is the same for both methods.
In your word process or photo program create a page that meets the dimensions of the template you were given with 0 (zero) margins on all sides. My template above (yours maybe be different) calls for 9.5 inches tall by 13.32 inches wide. This is for a 6 x 9 inch trade paper back (which is what most of you will be doing) with a 0.82 inch wide spine. There is also an allowance for “cuts” or “trims” from an oversize print job (about 1/4” all around).
You have to be precise! That's why mm are give in two places. They will reject something not to size.
The first thing we'll work on is the SPINE.
In a photo program like Gimp 2 or Paint simply start a new shape. It can be any size as we will crop it or you can size the canvas a little larger, say 1 inch tall by 13 inches long (we're doing this horizontally). Fill this page with an appropriate color or leave it white. Try and blend with your front and back covers if you can. You may need to open them up and use the “eye dropper” tool to sample the color, then use the fill tool on your page for the spine.
Pick a color for the text. Black or any bold color for white spines. White works best for any colored background.
With your text tool size it to about 1/2 the height of this 1” tall page and go all the way across. Then type in your book TITLE and AUTHOR NAME. Leave some spaces between title and name. Leave at least 2” on the right hand side, as that is where libraries will put their call number for the book. While public libraries may just put fiction books alphabetically, most colleges and universities use the Library of Congress catalog numbering system, even for fiction books. So leave some space so the label won't cover YOUR NAME.
This is what you should have. Now use the rotate, adjust or alignment tool to turn this from horizontal to vertical with the title at the top and the last letter of your name at the bottom. You now save this as SPINE and copy it to the clipboard.
Now go to your Word or Paint program, depending on what your printer wants as a final production tool, and copy this into the center of the page you have created.
This is how your Page in Word or blank canvas should look. Save this as SPINEIN.
Now it is simply a matter of inserting your front cover image and placing it all the way to the right and adjusting it to meet and slightly cover the oversize width spine.
Three should be a sizing tool in GIMP 2 and in the case of Paint you just drag the lower left corner to size the picture. If you make a goof simply close this, call back SPINEIN and copy the picture again and size it until you get it right. Then save this as FRONTCOVERIN.
Now you size a back cover picture in Paint or Gimp 2 that is 9.5” tall by 6.35” wide. You either leave it white if that's what you want or you use a fill color. Then you copy in a photo of yourself that is not professionally taken. Then you add bio text or no picture and a blurb about the book. Save this as BACK.
Now insert this picture from file and potion it so that it covers a little of the spine. Use the size or crop tool or any part of the image in Paint to size it. If it makes your photo look strange, crop the original BACK accordingly, save it as BACK2, remove BACK from your Word program or simply copy or import BACK2 into this form and adjust as required.
You should now as a print book cover.
Make sure your leave a white space three inches from the bottom and 1” from the spine for the ISBN bar code like your see above with that white space. If you don't they will cut into your back cover picture and text to put it there! That means you either live with that or re-do your cover and re-do the book design and get a new proof!
If they have a problem with the size, go back to your word processor or Gimp and adjust the size of the image a little. That's all they would want. A tenth of an inch here or there unless you really didn't understand the dimensions or they screwed up! As long as you crop or adjust the image equally on all sides it should not distort shapes. Small adjustments up and down or sideways may not harm things much, but if they really screwed up circles start to become ovals and you need to start the book cover process from scratch!
This should now give you a nice cover for your print book. Get a printed proof to make sure things are right. Adjust as required to get it perfect!