[OnePlus 6 Questions & Answers] flash713:
How did you (exact details please) go about installing SHRP recovery?

You mean you cannot hold down both volume +&- and your power button until you see fastboot mode the release buttons and then you should then be in fastboot mode .. A RED lettered "START" will be the first thing you should see..That's fastboot mode.. If I were you that's what I'd do then using fastboot from your PC boot twrp recovery or whatever recovery you're installing.. Make sure you right click where ever you have the recovery image you're going to boot on your PC and go to open command window here. I use Linux on my PC's so it's: sudo fastboot boot put the name of whatever your recovery image is that you're booting here and at the end should be ".img" and without the quotes...I always just go to where my recovery image is on my PC and right click, go to rename, then highlight the entire name and right click again and select copy.. Makes it easier sometimes the names are quite long then you can just paste it in your terminal window on your PC.. Used to you could rename the file and I used to rename mine to just "twrp.img" but now fastboot won't allow that. Throws an error says the names too short.. ? (hit enter key) Then you actually have more than one option to install twrp. I'll only mention one here to be less confusing.. Whatever recovery you're installing grab both the recovery image (ends in ".img") and the recovery installer zip for the recovery. Once you've booted your device to recovery using the recovery image flash the recovery installer zip using twrp or whatever you are using for recovery. Once you've flashed your recovery installer zip, if you want root, select reboot recovery, then flash the latest magisk zip , then reboot system. Now you should be good.

There is a chance it'll send you back to recovery instead of booting up to system. If it does there are no other options you must format data either using fastboot or twrp. That will wipe everything on your entire device. If you don't already use a USB thumb drive I suggest getting one or two. They're life savers!!

I wouldn't recommend using any toolkits. Fastboot and ADB are really easy to use and understand. In my opinion toolkits are more for some other devices.. It's good to learn the little bit you need to so that you know personally the right way to do things on your device. It may seem all complicated but it's really not. There are probably 500 YouTube videos on just about everything that has to do with fastboot and ADB and anything Android. They're very helpful for some people.. And XDA is full of bad ass tutorials on everything as well. I trust what's on XDA far as what developers post but many things users comment on all over xda , alot of times it's user error, it's their mistakes that cause the problems they're saying that they have. If you have any problems at all doing this msg me. I don't care who sends me personal messages or my user name is the same as my user name here on xda on telegram. Hmu there if you want. Hope this helps.

Easiest way to install fastboot and adb on windows PC: (takes 15 seconds!!)

https://xdaforums.com/showthread.php?t=2588979

Guide for it all: https://xdaforums.com/oneplus-6/how-to/oneplus-6-unlock-bootloader-flash-twrp-t3792643/page64

Another helpful link: https://www.xda-developers.com/adb-fastboot-any-directory-windows-linux/

If you cannot do what I typed then as a last resort: I'm not sure what model OnePlus 6 you have so careful there but msm tool is a way to revive your device. https://xdaforums.com/oneplus-6/how-to/tool-msmdownloadtool-v4-0-international-t3798892/page17