The basic process of installing TWRP on every phone is to boot into TWRP itself first so TWRP could overwite the recovery partition and replace the stock Android recovery with TWRP itself.
Boot into bootloader / fastboot mode
Make sure that your device's boot loader is unlocked. If not, please follow this tutorial on forum.xda-developers.com by user Mandrigan. https://forum.xda-developers.com/huawei-p9lite/how-to/guide-unlock-bootloader-twrp-root-t3405701
Boot into fastboot mode again. Type in the command
fastboot flash xxxxx.img
Where xxxxx.img is your TWRP image file name.
Immediately boot into recovery (make sure you don't boot into Android!!!)
While in TWRP, Press "Mount" and tick on "System" box. Then go to "Advanced" --> "File Manager" find a folder with the name "system". Go into that folder and scroll down until you find a file with something like "recovery-from-boot.xxx". Press on the file name and press on "Rename" and add ".bak" at the end of the file name and it should look like this "recovery-from-boot.xxx.bak"
It is recommended to flash either SuperSU or Magisk so TWRP stays and it doesn't get overwritten by stock Android recovery.
Installation is done and you should have TWRP as your permanent recovery!!