It is extremely
easy to install a Guest OS directly from a .iso file in Virtualbox. The first time you start up your Virtual Machine(VM), a wizard will pop up asking you to choose a .iso file to boot from. If you did not/forgot to choose a .iso file from that dialog, then you should either go to the settings of the VM and go to Storage, and for the IDE secondary master choose your CD/DVD image, or upon booting up your VM, go to Devices > CD/DVD devices. Once you're done installing, go to Devices > CD/DVD Images > Unmount CD/DVD Device.
Or you could just follow pags