If you aren't very good with electrical stuff you may not want to try this. You will need to open the main panel and make low voltage connections to a terminal block but there is also high voltage present. Best to shut off the power before fooling around.
There is a connector block on the lower left side of the main circuit board that has connections for a spa side remote. To turn items on and off you simply need a momentary contact to any of these connectors to toggle items on and off. Can't remember if you need a contact to ground or a voltage but your documentation should tell you that. The spa side remote can be configured to control most any of the pool or spa functions and not just the ones that may be hooked up at the moment. All you need is a momentary relay closure from the Elk to toggle things. Even if you don't have a spa the spa side remote connections are built into the panel and are a great way to control stuff.
At the moment I am only controlling an automatic fill line timer with mine but I ran enough wires to control anything I want. To be honest I found controlling most of the pool functions from a HA system to be overkill. The Compool controller can be programmed to handle most anything and can be expanded to handle additional features like fill lines or water features. If you need to control things from other places in the house or via phone then I can see the need for a HA panel but the built in panel is pretty complete.
I'm curious on what functions you want to automate with the Elk.