Google is your friend, mate.
The DSTwo site has everything you need. The download section has the firmware and a guide to setup the SD Card:
http://eng.supercard.sc/manual/dstwo/download.htm
I'm new here and have only just downgraded my N3DS today so take this with a grain of salt but, given that the newest system update is exploitable, you should be able to update to it then downgrade.
I used this Reddit post for help...
I seen a bunny rabbit get swept up and taken by a hawk yesterday in my back yard, it left a bunch of rabbit hair in grass. Wild to see it fly off carrying a rabbit.
The other night was watching tv in my screened in porch, and heard something walking on my porch roof, thought maybe it was a raccoon, so I grabbed my rifle and went out to look, it was just a duck, lol