Environment setup
It is recommended that Ubuntu desktop is used for the creation and management of signing keys.
Install snapcraft and login as the brand
Snapcraft is usually pre-installed. If it is not, you can install it as follows:
sudo snap install snapcraft --classic
Login to snapcraft as the Brand Account:
$ snapcraft login
Enter your Ubuntu One Email address and password.
If you do not have an Ubuntu One account, you can create one at https://snapcraft.io/account
Email: <YOUR-BRAND-EMAIL>
Password: ******
Login successful.
$ snapcraft whoami
email: <YOUR-BRAND-EMAIL>
developer-id: <YOUR-BRAND-ACCOUNT-ID>
Now you should be logged in with the Brand Account.