Go to https://mrminer.co/install and download the file from the install page.
After that you are going to need to flash img file into the USB drive. You are going to need etcher which can be downloaded from https://etcher.io/
First select the img file that you want to flash and that would be the latest version of Mr.Miner, then select the USB drive, lastly click flash.
After flashing, you are going to need to enter your email address into the email changer in the mining OS boot screen.