With the Grid environment variables, we execute the following command:
./u01/app/oracle/21.0.0/grid/bin/asmca
The ASM graphical configuration screen will appear, click on the "Disk groups" option.
We choose the disks that are going to belong to the diskgroup that we are going to create. They are disks that appear as "candidates" in the second column. We choose disks of the same size.
We indicate the name of the diskgroup, and we leave the default options, in this case we will create the disks with normal redundancy.
In this link you can find the reference to the ASM mirror and the Oracle diskgroups:
After clicking on the accept button, the screen with the new diskgroup created and the disks that compose it, as well as the redundancy of the created disks, will appear.
We can manually check the status of the diskgroups.
All these tasks can be performed manually. Check the official documentation for their performance.