Use a tool like BalenaEtcher or Win32DiskImager to write the .img file to your microSD card.
is also listed as a simpler custom option for Miyoo-based devices, though MiyooCFW is the standard for M3. Are you running into a specific error message black screen during your installation attempt? sup m3 custom firmware verified
If this firmware is for a vaping device (which the "M3" and "Sup" nomenclature strongly suggests regarding the Sup Mod / Vision mod scene): Use a tool like BalenaEtcher or Win32DiskImager to write the