Start with this in fastboot mode:
https://twrp.me/nvidia/nvidiahshieldtablet.html
Flash the TWRP - then you can search XDA to get a ROM to flash https://forum.xda-developers.com/shield-tablet/development/rom-lineageos-14-1-15-1-shield-tablet-t3950786
https://wiki.lineageos.org/devices/shieldtablet
Recovery: With the device powered off, hold Volume Down and tap Power, then release once menu is displayed. Tap Volume Down until recovery is selected. Press Power to select.
Download: With the device powered off, hold Volume Down and tap Power, then release once menu is displayed.