Ok... You need to wire the Heatlink into the boiler. That's the square, white, flat box.
When you open the connection section you'll see 13 ports. You'll only need 4.
From left to right;
N - connect this to mains Neutral
L - connect this to mains Live
You can feed the L and N from.the L and N off the boiler mains connection - just run a wire from the L and N of the boiler to these.
1 - Ignore
2 - connect this with a short piece of wire to port L. This will provide live to the switched live of port 3
3 - this need to be connected to the switched live on your boiler. This is what tell the boiler that you are requesting heat and turns the heating on
This will power the Heatlink. When your thermostat calls for heat, it activates the Heatlink and connects live to port 3 in the Heatlink which then puts power to the "call for heat" on the boiler and your heating turns on.
Plug your thermostat part into a wall socket and pair it up with your Heatlink.
Now the above is the very basic and simple setup. It just uses the learning Thermostat part of the setup to turn the heat on and off.
If you want to use Opentherm, that's what the OT1 and OT2 ports on the Heatlink are for. Apologies but I ain't 100% sure how to wire that up but it's likely a case of connecting your Opentherm ports on your boiler to the OT ports on the Heatlink.
Edit - I should add that the above is a template installation. Without knowing the wiring connections on your boiler, it's hard to give an exact connection diagram