How can Turn off Apple Pencil

There is no need to turn it off (and it can't be turned off). However, when you stop using the Pencil, it goes into a low power mode, and disengages from the iPad in terms of bluetooth connection. At the moment you pick it up, it will reconnect to the iPad.

You cannot power off an Pencil

That stated and FYI,

something Apple will never, EVER divulge to it potential iPad Pro/Pencil customers/users unless the potential customer/user is smart enough to ask about,

Due to the “always on and active/standby” nature of the Pencil and, also, due to its very tiny rechargeable Li-

ion battery ( smaller than the eraser head on a REAL wooden pencil ), the Pencil needs to be constantly charged up to some significant charge level ALL THE TIME, even when not using the Pencil for prolonged periods of time.

The Pencil needs to be kept charged to a minimum of 5-10% all the time.

If the Pencil battery is allowed to drain down to 0% and allowed to stay in that flat condition for more than a few weeks, or so, OR EVEN LONGER, that very tiny little Li-ion battery is too small to keep at a 0% state for a long period of time and it will fail and the Pencil will be dead and you may have to “pop” for another $99/$129 Pencil, once again!

Apple only has a one-year warranty on any Apple accessory items.

You need to keep your Pencil charged all the time, even if it is only to 10%-15% charge. It must keep some charge level in that tiny Lithium-ion battery at all times!