A guide to talk you through installing cores manually on RetroArch. We're using *.so files which must be located in RetroArch's download folder. Please ensure you have the latest version of RA (NOT the current Google Playstore version!). Don't panic if the core isn't named or listed where you expect it, restart RA and it will be named appropriately.
RA Download link: https://www.retroarch.com/?page=platforms