You can dump the DSiWare from your 3DS using GodMode9, see the
page on 3ds.hacks.guide for how. (But choose
Dump CXI/NDS file
instead of
Build CIA (standard)
)
Then you can either use
TWiLight Menu++ to run the DSiWare from SD or
NTM to install to the DSi Menu (either SysNAND or
hiyaCFW).
If you haven't modded either of them yet simply follow
3ds.hacks.guide for the 3DS and
dsi.cfw.guide for the DSi. On the 3DS you'll need full CFW, on the DSi you can just use Memory Pit if you want, though I'd personally recommend getting Unlaunch as that way you can load TWiLight Menu++ on boot if you want or it makes it less risky if you choose to use NTM on SysNAND.