You don't turn it back on. You turn it off, leave it off then connect it to the computer while holding either the Top button (on an iPad with Face ID) or the Home button (on other iPads) and keep holding that button until it starts and displays the recovery screen. Then follow the other instructions in the link.
See one of the following (depending on your iPad model) from If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
iPad with Face ID: Press and hold the Top button and either volume
button until the power off slider appears. Drag the slider to turn off
your device. Connect your device to your computer while holding the Top
button. Keep holding the Top button until you see the recovery mode
screen.
or
iPad with Home button: Press and hold the Side (or Top) button until the
power off slider appears. Drag the slider to turn off your device.
Connect your device to your computer while holding the Home button. Keep
holding the Home button until you see the recovery mode screen.