In the Finder, copy the documents you want to protect to the disk image. It mounts its disk icon on your desktop and in the Finder sidebar.
Make a note of the password and save it somewhere handy.
You can then restore that disk image to another volume.
For example, if a USB device or volume is 80GB with 10GB of data, the disk image will be 80 GB in size and include data and free space. With the Mac’s Disk Utility - which is found in the Utilities folder inside the Applications folder on your Mac’s drive - you can create a disk image that includes the data and free space on a physical disk or connected device, such as a USB device. Just create an image of that drive and store it. If you want/need to keep data from a Mac, you don’t need to keep the actual hardware around.