Download Any Gun Plugin.
Then Extract It And Follow The Given Instruction:-
1) Put The .amxx File On cstrike/addons/amxmodx/plugin
2) Put The models At cstrike/models
3) Put sounds Folder At cstrike/sounds/weapons
4) Put sprites File At cstrike/sprites
5) Then Put The Plugin Name In Plugin.ini cstrike/addons/amxmodx/config/plugins.ini
If You Need More Help So Lemme Now