How do I activate the debug mode?

Basically there are two options to enable debug mode.

Via configuration.yaml

Add the following lines to your configuration.yaml file and restart Home Assistant.

logger:
  default: warning
  logs:
    custom_components.better_thermostat: debug

Via services

Open your Home Assistant instance and show your service developer tools.

Another option is to use the service logger.set_level. Go to Services under Developer Tools, switch to YAML mode and enter the following YAML.

service: logger.set_level
data:
  custom_components.better_thermostat: debug

Click Call Service, no restart of Home Assistant required.