Fitting instructions for the brass fittings:

These are from Richard, who manufactured the fittings

I recommend removing the carburetor top and then tapping the thread required, 

BUT others will simply slap some grease on the tap then gently and carefully, making certain the tap is square to the hole,turn the threading tap a couple of turns, remove the tap, clean off the swarf,put some fresh grease on, thread some more until the job is done.

At the very least, remove the filter nut at the top left side of the unit and block the access. You can also gain access to clean out swarf.

A small amount of thread tape, probably one wrapped turn will be sufficient, then thread the new fitting into place, with your 11mm or 7/16" wrench.

I prefer to also use a hose clamp on the new fitting, even though the barbs will hold the plot, no worries.

The carb metal is quite soft and I found it easy to tap with the top in place following Richo's care instructions.