The guages will come with instructions BUT I'll advise you to get an electric type oil pressure guage, not what's called a "manual" guage that'd require running a plastic oil line into the interior of the car.
The manual type oil pressure guage might be more accurate BUT it's something that I wouldn't recommend for most as the lines can leak and/or rupture in time. Those are best for race cars or hot-rods where the installer doesn't mind the additional hassle factor. The electric oil pressure guage will be accurate enough for most people and it should come with extra fittings too.
The oil pressure sender is on the front side of the engine, center block area above the oil pan and under the intake manifold.