How do I lock my iPad.

Locking an iPad as simple as clicking the physical Top (Sleep/Wake) button.

The iPad User Guide provides information on locking and unlocking your iPad - and how to configure the lock function.

Control access to information on the iPad Lock Screen - Apple Support

Wake and unlock iPad - Apple Support

Manually … w/ a single “click” of the “power” button.

Lock in what way exactly?

If you use a passcode, the iPad locks automatically when the screen turns off.

If you lost the iPad or it was stolen, follow the steps in the support document linked below to set it to lost mode and "lock" it.

click here ➜ If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support