There is a twrp port here in the forum. But I would try to learn all this step by step. How to do a backup, how to reinstall stuff … so that you have all this when you need it. The best is you just try it and write a howto while doing so 
I don’t own a FP2 so I don’t want to give you wrong information. But if you read the wiki about building your own non-google images, you can also see how these images get flashed. It’s the same with other images.
Normally you put your phone into the bootloader/fastboot mode and unzip the images (FP2-gms36-1.1.7-img.zip, the twrp image) on your computer and flash your phone from there.