you pop the breather cap off the diif and 1/2" hose will push on and lock it there with a hose clamp. I dont like this way though, 1/2" hose is major overkill for a breather.

What I done is pop the breather cap off, tapped out the hole to 1/8"BSP (no drilling required, the tap is a perfect fit in there). use a strong manget on the side of the tap and most of the filings will stick to the tap and not fall into the diff, then drain and flush the diff when your done to remove any filings that did get in there.

you can then use a brass barbed hosetail and attach any size hose you want. 1/4" is the best for it. You could also use nylon airline (like airlocker hose) and pushfit fittings, looks very neat when done.

for the other end of the breather I used an airline silencer, its a fitting with a honeycomb type of filter on it.