[OnePlus 6T  Guides, News, & Discussion] Causical:
Ok you could refer to [MENTION=9036808]TENlll423[/MENTION] but excuse me for not feeling like being redundant right now. You don't need a guide, you just need a simple understanding of how your device is set up.

Just think about this for a second, you have 1 phone with 1 slot with 1 way to flash right? Things get pretty obvious at that point you just have to go into recovery (which flashes software for you) and flash what you want on your phone right?

Well what do you want? You know you want a ROM (operating system), you know you want to keep your custom recovery (TWRP) and you know you want root (Magisk)

So then all you need to know is which of these am I supposed to flash first, because is there a specific order?

The answer would be YES because flashing one thing before or after another thing will effectively overwrite something you may need. The order that you flash your files is specifically based on what will get overwritten if you DON'T flash in a specific order.

That's why we flash: {segment #1}

(1)•ROM first>>>>because it overwrites both TWRP and Magisk (which means it WILL replace any custom recovery or Magisk you have currently installed with stock recovery and boot image =not what you want)

(2)•TWRP second>>>>because it overwrites Magisk but NOT the ROM (meaning the ROM you just flashed sticks but if you had Magisk it gets erased)

(3)•Magisk third>>>>because it technically doesn't write over anything you need (ROM nor TWRP) except maybe a custom kernel sometimes (some custom kernel developers recommend you flash your custom kernel after you flashed Magisk, but if you're not wanting a custom kernel you don't need to worry about it)

Now back to the beginning, You know that you don't in fact have a phone with just 1 slot, you have a phone with 2. So you have to do the same thing I just described for both slots.

Here's the difference, you already know the correct order to flash your files, now it's important to know where those files are flashing to in relation to your slots and that goes like this:{segment #2}

(1)•ROM will flash ALWAYS to the slot that you are NOT currently booted on. (whether you are booted into TWRP or your phone system) {[this means that if you are on slot A, you will be installing a ROM for slot B and vice versa]}

(2)•TWRP installer zip will ALWAYS flash to BOTH slots at the same time {[this means that now your opposite slot will have your new ROM and TWRP and the slot you're currently booted on will still have your old ROM and TWRP]}

(3)•Magisk will ALWAYS flash to the slot that you are CURRENTLY booted on (whether you are in TWRP or your phone system) {[this means that if you flash Magisk NOW before rebooting to your other slot, you will be flashing Magisk to the slot that has your old ROM and TWRP]}

**Note about Magisk**

I personally recommend flashing Magisk after EVERY TWRP flash because it helps with ensuring that you don't run into encryption or booting issues.

*Another important note, you don't need to worry about manually switching between your slots. Whenever you flash a NEW ROM it will trigger the bootloader to tell the phone to automatically boot the new slot. (So if you were in TWRP flashing your files, simply rebooting after all your initial flashes from {segment #1} would automatically reboot you to your other slot because you flashed a NEW ROM. So then you would just continue flashing for your other slot.)

**IF you so desire to have both of your slots completely identical, when you have already done your flashes for both slots (2 times per each file you flash on different slots) you will need to reboot TWRP once more (the 3rd time) because the last time you flashed TWRP on your second slot it overwrote the Magisk you had already flashed. So flash Magisk again.