Transfer the custom ROM .zip file and the OpenGApps .zip file to your phone's internal storage or SD card. On the TWRP home screen, tap . Navigate to and select your custom ROM .zip file. Tap Add more Zips and select the OpenGApps .zip file.
If you need to revert to stock, download from SamMobile or use Frija. sm-g900t custom rom
Go to "Wipe," choose "Advanced Wipe," and select Dalvik, System, Data, and Cache. Install ROM: Transfer the custom ROM
A clean flash is mandatory when switching from stock firmware to a custom ROM. In the TWRP main menu, tap . Select Advanced Wipe . " choose "Advanced Wipe