Updated June 3 2015
1. Make sure you have all png images in both the normal and the retina version.
2. Make sure the retina versions are exactly 2x bigger than the normal versions
3. Create 2 zip files. 1 for the normal images and 1 for the retina versions
4. Got to http://spritegen.website-performance.org/
5. Upload the normal zip file. Choose offset of 25px. Copy the css and download the file
Update: because of some Safari bug it may be better to use 50px for the normal and 100px for the @2x sprite
Drag the images to the generator. Choose padding of 5px. Choose a vertical layout. Click downloads and save the sprite and the stylesheet.
6. Upload the retina zip file. Choose offset of 50px. Do the same for the retina images, but choose a padding of 10px. Copy the css and download the file
Note 1: Read More