ok, first thing to do, is buy a decent boss, that is designed specifically for the Impreza.

Cant recommend this one enough:

Works Bell Standard Boss Kit 115 Subaru

Yes from Japan, but worth every penny. It comes with all OEM connectors to allow horn to work easily and to stop the Airbag light coming on.

So once you have the hub, and your wheel of choice, its a case of:

Undo screws on side of wheel to release airbag.



Loosen centre nut, and swing on the wheel like a mad man, until it comes loose. Then unwind the rest of the nut (stops you getting a wheel to the face).

Then fit on your wonderful Works Bell boss kit, feeding through any wires needed (generally following the pictured instructions), in my case, just the single Horn wire.



Get your new wheel ready, in my case some JDM megabling in the form of a Key!s Racing 350mm buckskin steering wheel.



Screw it into place using provided bolts



Stand back and admire



Go Enjoy!

 

And dont forget to tell your insurance company about the removal of your drivers airbag, or your insurance will be void!!