@David_Maas. It is not too complicated, but you would have to piece together instructions from “here and there”, depending on what hardware you have available. Short summary as follows:
- Install and configure Home Assistant on preferred hardware. Mine runs in docker on a Synology NAS. https://www.home-assistant.io/getting-started/
- Install and configure a MQTT broker on preferred hardware. I run Mosquitto in docker on a Synology NAS. There is a broker app for Homey that I haven´t tried but it should do the job from what I´ve heard.
https://mosquitto.org/download/ OR https://apps.athom.com/app/nl.scanno.mqttbroker - Install MQTT client app on Homey and configure it to connect to the broker.
https://apps.athom.com/app/nl.scanno.mqtt - Install MQTT hub app on Homey and configure it to for Home Assistant discovery. How-to-instructions inside the app setting.
https://apps.athom.com/app/nl.hdg.mqtt - Set up an integration to the broker in Home Assistant configuration. Can be done in the GUI and is fairly simple.
- BADABAM! Your Homey devices should be discovered and appear as entities in HA.
Let the fun begin!
Good luck, and don´t forget to support the developers 
