Format your SD card as FAT32. (Use a fast, trusted brand. The SD card sent with your Miyoo Mini is slow, and will likely ruin your experience and data.)
Unzip the .tmp_update folder and its contents to the root of your SD.
Boot up your Miyoo Mini and follow the on-screen instructions to get started.
Your Mini will turn off once installation is complete. (Don't delete the .tmp_update folder, it is now part of the Onion installation.)
Copy your Bios files into the BIOS folder.
Installation (Upgrade):
(If coming from <= V3.09, backup your saves, you will have to copy them afterwards in the Saves folder of your SD)
Delete all folders from your SD except BIOS, Roms, Saves and Themes.
Unzip the .tmp_update folder to the root of your SD.
Boot up your Miyoo Mini and follow the on-screen instructions to get started.
Your Mini will turn off once installation is complete. (Don't delete the .tmp_update folder, it is now part of the Onion installation.)