Gimp comes with a template built in. File > New > Template > CD Cover
Copy the background layer to a new layer, then turn off the visibility for the background layer.
Select the new layer, and using the elliptical select tool, starting at coordinates 0,0 (shown in status bar) and holding the shift key down, drag to create a circle that touches all 4 sides of the image. Then use Edit > Select > Inverse to invert the selection. Then select Edit > Cut.
This leaves you with a circle where you can put your images and text. You may need to cut a smaller circle from the centre of the image for the spindle hole. You do this by finding out the diameter of your unprintable area on the cd (22mm on full face) and repeat the selection process as described above, but starting at x/y coordinates of 49/49 and drag the selection until the selection size is 22 by 22 mm (the selection size is shown in the status bar). Cut as before.
Now make new layers for your text and images, so that you don't break your circles ! You can also save it as a template for later use.
How you print it out is related to how your printer works.