1.解锁 unlock the bootloader;
2.刷twrp (flash twrp)
从这个帖子里下载twrp，感谢mauronofrio（download twrp from this post, thanks for mauronofrio’s hard work）
TWRP 3.3.1-25 Unofficial by mauronofrio
https://forum.xda-developers.com/k20-pro/development/recovery-unofficial-twrp-xiaomi-redmi-t3944363
用fasterboot刷recovery(flash twrp recovery via fastboot)
fastboot flash recovery twrp-3.3.1-25-raphael-mauronofrio.img
进入twrp recovery (reboot to twrp recovery)
注意：不能直接重启进入系统，否则twrp将会被替换（IMPORTANT： After flash twrp, DO NOT REBOOT TO MIUI DIRECTLY, OTHERWISE TWRP WILL BE REPLACED BY MIUI RECOVERY）
fastboot boot twrp-3.3.1-25-raphael-mauronofrio.img
在这个帖子里的地址下载K20Pro 的10.0 vendor和LOS17.0 ROM，用MTP上传至手机，感谢abhishek987