Creating the Image
If you want to make a DVD image in Mac OS X, begin by finding the data which you wish to convert to this format, and group it together in a single folder in a location that you will easily remember in a few minutes time.
Next, open Applications > Utilities > Disk Utility, and use New Image to create and name a new .DMG file. When this is created, it will be automatically mounted, so using the Finder drag the folder you created previously onto the new .DMG, which you should find situated on the left-hand side of the Finder.
Back in Disk Utility, you should see that the data you added is listed under the .DMG image. With the image selected, go to Images > Convert… and Save the image with a meaningful name, adding the file extension .CDR, selecting DVD/CD Master as the Image Format.
Currently, you have a .DMG file, and this can be burnt to a recordable or rewritable CD or DVD, depending on the size of the image file and the capacity of the disc. However, if the data you are creating a disc image for is something that might be read on a Windows computer, you will need to convert to the .ISO format.
Fortunately, this is what the .CDR is, but you will need to change the file extension to reflect this. In the Finder, use the eject button to unmount the image, then right-click and select Get Info. Here, find and expand the Name & Extension field and add the .ISO extension. You will be asked to confirm this action, and once you have done so the image will be ready to use on a Windows computer.