Installing the new module
Before a new addon module can be installed, the module list needs to be updated.
This can be done from the Apps top menu and the Update Apps List option. It updates the module list, adding any modules that may have been added since the last update to the list. Remember that we need the developer mode enabled for this option to be visible.
Make sure your web client session is working with the right database. You can check that at the top-right: the database name is shown in parentheses, right after the username, a way to have the server limited to using only the intended database. Until Odoo 11.0, this was done with the additional option --db-filter=^MYDB$. Since Odoo 11.0, this is not required, as by default -d (or --database) automatically performs that filter.
The Apps option shows us the list of available modules. By default, it shows only the application modules. Since we have created an application module, we don't need to remove that filter to see it. Type todo in the search and you should see our new module, ready to be installed:
Now click on the module's Install button, and we're ready!