1. Unzip the download and add the module folders (master_hub_core, master_hub_mail, plus master_hub_tasks and master_hub_mail_ai if you bought them) to your Odoo.sh repository, then push to a staging branch first.
2. Create the encryption key (any machine with Python): python3 -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())". Keep a copy in your password manager.
3. In the Odoo.sh shell, save it as a key file in the data directory: mkdir -p ~/data/master_hub_mail && chmod 700 ~/data/master_hub_mail, then write the key to ~/data/master_hub_mail/key and chmod 600 it.
4. In Odoo: Apps › Update Apps List, search "Hub Mail", Install.
5. Test on staging, then merge to production and repeat the key file step there.

Upgrading later? Pause the "Hub Mail" scheduled actions first and switch them back on afterwards.