Same as you flash any other rom. Reboot to twrp and flash the rom then flash twrp.
Also if you flashed twrp installer it patches the boot.img of both slots so you will have twrp on both slots not just slot which is more than likely your situation.
What you should definitely do though is flash stock boot.img to slot b and then patch it with twrp because if you flash a rom with otas it won't work as the boot.img on slot b gets destroyed when flashing twrp.img.
If you flash stock boot.img to slot b and then flash twrp you will have a bootable boot.img on both slots with twrp also installed on both slots.
This probably should have been asked on the pe thread itself. I don't think there was a reason to open a new question you probably would have received more responses on the applicable thread
