How do I enter twrp recovery?
Installing a recovery and flashing your rom

Download your recovery.img or wahtever the name is to your fastboot folder.
adb reboot bootloader
fastboot flash recovery recovery.img
fastboot reboot recovery
Now use your recovery to flash your rom.

At this point you may not be able boot into your rom if you do not have S-OFF.But give it a try. If it works then forget the next few lines.

Copy the boot.img file from inside your rom and copy it to your fasboot folder.
adb reboot bootloader
fastboot flash boot boot.img.

Now you can reboot to boot into your rom.