To turn off a Tesla vehicle manually while inside, follow these steps:

1. Shift the Tesla into Park.
2. On the touchscreen, go to Controls > Safety and Security.
3. Scroll down and tap on the "Power Off" option.
4. The car will power down after a few minutes; do not touch the brake or open the door during this time.
5. To turn it back on, just press the brake pedal.

Note that Tesla vehicles are designed to automatically turn off (go into sleep mode) when you walk away or after some time parked. Manually powering off is usually not necessary unless the car exhibits unusual behavior or you are working on it. Completely powering off so it won't turn on again without intervention requires disconnecting the 12-volt battery, which is not generally recommended without expertise. In short, just put the car in park and walk away for normal use; manual power off is available through the screen if needed.