To perform a snapshot (a copy of your disk at a given time), you need to be in the management page of the disk in question (“Disks” tab, then click on the disk in question).

You can then either us the link on the right of the page to “Create a snapshot on-the-fly”, or scroll down to the Snapshots section to create a snapshot.

Just enter a name of the snapshot so that you can identify it later (for example, the version of your data)

You can make as many snapshots as you would like.