Output to Video Card?

What do i need to do in order to get the output that i see on screen to a video card like Blackmagic Decklink or AJA Kona?

Do i just buy the card and install and it will auto output to it or do i have to do some special converters or tweaks?

Please do enlighten me, as i would like to get the output of the screen onto a sdi connection (running at 25 fps)