For MS-Windows users, there are numerous .EXE files available that simplify the drivers/tools/unlocking/TWRP/root process. It can get somewhat complex on MS-Win, thus those ever so "helpful" .EXEs.

The problem is, one doesn't know what is inside those binary .EXE files, or what else they just happen to install on both the PC and the phone while doing the biz.

From a Linux system, or even just a LiveUSB stick, one can see the entire process in action, and realize that it is much simpler than people may lead one to believe.

First, install the Android "fastboot" command on the Linux box. Just typing it at the prompt on a Linux Mint (or Ubuntu) system should give back the command to install it if not already present. You'll also need to locate/download a suitable TWRP image file for your specific phone.

Connect the phone to the Linux box with a USB cable.

Now, put the phone into "bootloader" mode. The exact method varies by brand/model, but usually is something like: power off the phone, then hold Vol-Up while pressing the Power button.

From the Linux box (as root):

fastboot devices ## See if it found your phone.
fastboot oem unlock ## Wipes the phone, unlocks the bootloader

After the phone resets/reboots, put it back into bootloader mode again, same as before. Then do this to flash the TWRP image, and boot directly into it for the first time.

fastboot flash recovery NAME_OF_TWRP_RECOVERY_IMAGE_FILE
fastboot boot NAME_OF_TWRP_RECOVERY_IMAGE_FILE

Enjoy.

Another useful thing to do, is enable "Developer" options on the Android device. This is normally done by going to the "Settings" app, then to "About Phone", and then repeatedly tapping on the "Build number" item until it says you are now a Developer.

The result of that, is a new "Developer Options" menu entry under Settings (one level up from "About Phone"). Go there, and enable "Advanced Reboot" and "Android Debugging". This will then allow you to get into TWRP more directly from the power-off confirmation dialog (press and hold power button, select Reboot, and observe the new menu that then appears).

The "Android Debugging" thing enables use of the "adb" command from a connected PC to manage the phone (upload/download files, issue commands, and generally poke around). The first time a particular PC is connected, the phone will pop up a security confirmation dialog to permit/deny access to the phone from that PC.

I cannot recall, but you may even find it necessary to do this step above before anything else can be done (eg. unlocking, flashing recovery, ..).

Cheers