This will explain how to create an ISO image from a CD/DVD using Brasero in Linux.
First, insert the disc from which you want to make an ISO image. Wait until, and make sure, an icon for the disc appears on your desktop, indicating the disc has been read and mounted.
Next, open Brasero and select the Disc Copy function.
On the Copy CD/DVD dialog, change Select a disc to write to to ISO by clicking on the drop-down itself (not the properties button) and choosing Image file.
Click the Properties button and choose a name and location for the output ISO image and then click Close.
Verify your settings and then click Start copy.
Brasero will start creating your disc image, which will be quite large — up to the full capacity of your media (4.7GB for single-layer DVDs, 8.5GB for dual-layer).