nds2elf.py uses LIEF and template.elf to form a new binary. LIEF is available via pip:

pip3 install lief

DSi and DSi-enhanced titles currently need to be demodcrypted prior to running. This can be done with twltool and

twltool modcrypt --in sys_menu.nds --out sys_menu.dec.nds

Once decrypted:

python3 nds2elf.py sys_menu.dec.nds regs_arm7_list.txt

The output in this case would be sys_menu.dec.nds.elf.