Posting the steps to install the recovery, so everyone knows.
1-Connect your phone into fastboot mode using pc.
2-Open adb tools command prompt.
3-Type fasboot flash recovery recovery.img (recovery.img is the twrp you just downloaded, should be in the same folder from where you opened the command prompt).
4-Type fastboot boot recovery.img
5-TWRP will open. It might not detect your internal storage and ask for a password.
6-Back your storage up.
7-Then, go to wipe and wipe everything. After this your storage will be empty.
8-Since there's no os, install a custom rom and reboot.

Cheers