I can give you an overview of how to / why do it. But it would be much easier if you mentioned which android version you're running, and then find a copy of that boot loader with twrp already injected.
twrp is no longer in it's own recovery partition, so 'flashing' it is not the same. It has to be ram disk injected into the boot partition. To do that, you actually need the / a device with twrp already working, as it's an option in twrp to ramdisk inject the twrp image.
In some cases, if you have a version of fastboot that will boot certain versions (that are bootable) of twrp, then you can use it to do the 'ramdisk' inject twrp.
As said, mention the version of android you are running, the boot partitions with twrp are around for all of them.
So what you'll have to do, to use the attached zip file, is to be; 1) rooted, 2) use magisk manager to create the boot partition with magisk installed, 3) use qfil to flash the newly created boot partition with magisk (be sure to save off your original boot partition first), 4) flash the attached zip file with magisk manager which will inject twrp into boot partition.
You'll get a message after doing above saying you're now 'unrooted'. But to fix that just use twrp to flash magisk zip.