They work just like regular sights. You put the front sight in the small notch at the bottom of the buckhorn "horns".
See this. new marlin 1894 shoots way high - Shooters Forum
so i went out yesterday and shot my rio grande for the first time with its buckhorn sights at 50 yds. I couldnt get it to hit in good groups consistently. I'm not familiar with buckhorn sights and not sure that i'm getting the sight picture correct. Any suggestions. Oh and the gun hates monarch ammo