You want to have a 3DS firmware version of 10.7 or lower in order to downgrade, then follow this guide to set up Homebrew, downgrade your system to 9.2, and set up custom firmware so that you can play 3DS ROMs in .cia format.
Sky3ds+ supports only 3ds roms. If you want ds roms, buy the r4i gold pro. Also, both are on ebay, though the sky3ds+ is about 120 dollars or 90 pounds, but the r4i gold pro can be found for under 20 dollars, or around 15 pounds. But it's easier just to softmod your 3ds for 3ds games. DS emulation is possible, but it's experimental with no sound.