In Goldsource (HL1), make a new folder within the Half-Life directory. Take a copy of the liblist.gam file from Valve, place it in your mod folder and edit accordingly.
If you're using Steam HL, restart Steam and your mod's name (as set in your liblist.gam file) will appear in the games list. If you're using Xash3D or WON Half-Life, your mod will appear in the Custom Game tab in the main menu.
Once that's done, you can throw whatever files you like into your new mod directory.