You have to have flasher 3.5 installed, the rootfs image .bin and if necessary the eMMC image .bin downloaded.

Tell flasher to flash, it'll say no device found, waiting.

Connect the phone via USB while holding the U key.

Flasher will detect it, flash it and say done, the phone should then boot automatically.
Leave it to boot. Unplug and you're away.

READ THE WIKI!