I’m going to assume you have a running instance already. Mine is called test-instance. If you don't please read How to create an instance.
Create the volume by clicking Compute -> Volumes -> Create Volume.
Mine is called test-volume, will be 20 GB large and use the regular storage tier.
After you have created the volume it is time to attach it to your instance. This can be done with the context menu of the test-volume row. Click manage attachments.
Choose the instance you want to attach the volume to, test-instance in my case. And choose Attach Volume.
In the volume list you should now see something like this:
Especially the "Attached to test-instance on /dev/vdb" part is important. This is the device name we will be using in our instance.
It is now time to log in to the instance we have attached the volume to. See How to log in to your instance if you don't know how to do this.
To be able to use the volume we will first need to format it. I will use ext4 as my filesystem. There are also other filesystems to choose from. My instance was attached on /dev/vdb, so that is the block device I need to format. Replace /dev/vdb with the device your volume is attached to.
Log in with ssh and execute the following commands:
$ sudo mkfs.ext4 /dev/vdb
sudo commands might give the following error:
sudo: unable to resolve host test-instance
This is harmless and can be ignored. If you want to get rid off it, you can add the hostname to /etc/hosts:
$ echo "127.0.0.1 test-instance" | sudo tee -a /etc/hosts
The volume is attached to our instance and we have formatted it. Before we can use it we need to mount it. If you do not have experience with mounting filesystems in Linux, Ubuntu has a good explanation.
We need to make a mount point to mount our volume in:
$ sudo mkdir /mnt/test-volume
We are going to add this mount point to /etc/fstab, so it will be mounted automatically on startup. Do this using your favorite editor. Mine is vim.
$ sudo vim /etc/fstab
And add this line to the end of the file:
/dev/vdb /mnt/test-volume ext4 defaults 0 0
Save and close (How to exit Vim). This will automatically mount the volume at startup, but we still need to mount it manually now:
$ sudo mount /mnt/test-volume
This will mount the volume. Any data you now save to /mnt/test-volume will be stored on the volume.