You need to have the amiga connected to either an Amiga montior or a TV trough the RGB port (scart or RF-mod). Then you install the video drivers, either Picasso96 (free) or CGFX (commersial). After that you set screenmode in workbench to use the new screenmodes on the videocard.
You'll however have a bit of an issue with all none workbench software and games as they'll typicly run on the native Amiga screenmodes. There's where a scandoubler comes in handy as it allows these modes to be visible on a normal VGA monitor (easiest would be to install it and then have an external monitor switchbox where you manualy change between the modes as you already bought a videocard with no internal switching capabilities on the A2000).
with no scandoubler a dual screen scenario is needed with both a VGA monitor connected to the videocard and a native monitor/tv connected to the Amiga rgb port.
