How to turn on and off the Apple Pencil 1st generation?

You don't. There is no way to turn it on or off. It is always on, that is why it is important that you keep an eye on the charge of the Pencil and ensure that you keep a charge on it all the time to prevent the battery from failing.

Check out these two support documents as well for additional information about the Apple Pencil.

https://support.apple.com/en-us/HT205236

Use Apple Pencil with your iPad - Apple Support