I'm putting the original instructions below: the main edit is "Format Data" instead of "Wipe Internal Storage".
Additional notes: I couldn't get MTP to work because I'm using a Mac, so I had to use a USB-OTG cable. Also, I flashed the TWRP recovery to make it easier for flashing going forward...
I successfully did this yesterday. The critical part is realizing that none of the existing recoveries (that I found) can decrypt the encryption (I was on stock 6X MIUI 10), so your only choice is to "format data" to remove the encryption.
1) to make things easier, I put everything onto a usb...
I installed twrp img via USB OTG, but I still had to choose "format data" -- the "wipe internal storage" still didn't work.
After formatting data, I was able to install the latest image (still via USB OTG).