Place the ns folder in your half-life folder and replace all when asked.  This is a patch to apply to existing installations of NS 3.2 or later.

The Half-Life folder can be found here:
steam library -> Half-Life -> right click -> properties -> local files -> browse local files

FOR USERS WITH CUSTOM HUD FILES:
Any custom hud files in your existing installation won't be changed when installing via this patch. To use your old personalized hud set the hud to default in advanced options (or hud_style 0) and restart your game.  The map style can also be set in the advanced options or via hud_mapstyle.  Note that these options won't work as described in the menu since your files are custom. The same applies to cl_ambientsound if you modified your sound files.