With the tablet still powered off, boot into recovery by holding . Once you see the Samsung logo, you can release the power button but keep holding the other two until TWRP appears. (Note: On some newer bootloaders, you might need to use the 3-button method from a powered-off state to avoid a recovery bootloop).
Keeping the original Samsung look but with significantly better speed and less junk.
Connect your tablet to your PC while still in TWRP to transfer the custom ROM .zip and GApps .zip files to your device's internal storage or external SD card. In the TWRP main menu, tap . Navigate to and select your custom ROM .zip file.