Just wondering how the valve needle gasket installs on a Holley NH carburetor.
Do you mean the 6219 gasket? It slips over the end of the needle seat part 6212. You screw in the 6212 into the carburetor body. There is no gasket for the richment needle part 6208B.
If you have an early '25 with the older mixture screw, here is the applicable drawing (all credit to Martyn Vowell for both drawings):
You forgot one....
The Side Drain...which is a little different all the way round. In fact if you didn't know better you'd think you were looking at a Kingston carb.