here is how you do it its pretty simple
first create a folder mods in the folder where your game is
next download a mod for example gungame there are few ways the mod will come
sometimes it will already be in a folder like gungame and if you open it there will be a file gungame.iwd
sometimes its gungame.rar rename it to iwd
sometimes when you open the mod there will be a folder maps add it to winrar then rename it to gungame.iwd next create a new folder called gungame and put gungame.iwd in it
then in console if youre not playing a map yet write fs_game mods/gungame;vid_restart
if youre already playing a map write fs_game mods/gungame;map_restart
btw you gotta be host to play mods hope i helped if not just ask