Use the attached one.
1. Boot in to TWRP
2. Wipe > Format data (beware that you will loose all your data)
3. Hit the home button in TWRP and reboot to recovery
4. Copy RemoveEncryption-P20Pro.zip and flash it
5. Hit the home button in TWRP and reboot to recovery
6. Wipe > Format data
7. Hit the home button in TWRP and reboot to recovery
8. All set done. Now you can copy any files that you want to flash (i.e EMUI 9.0 and HuRUpdater 04 etc)
Hope it helps.
Thinking of doing this decryption. However my question is, after I decrypt, and when I flash the next firmware update, will my data get encrypted again?