How do I do it?! Continuity

Follow the steps in the link you posted for whatever part of Continuity you want to set up.

To use your iPad as a second screen, follow the steps in the section of the link you posted titled "Side Car"

Or see here: Use your iPad as a second display for your Mac with Sidecar - Apple Support

Continuity is a collective concept, not something you switch on and off with a single toggle. You learn how to use each feature (including the one you asked about) by clicking on the blue link for each one described in the link you provided and read bout how to use each one.

To use the Sidecar feature with iPad/Continuity/Mac,

From Apple.

'Sidecar Supported hardware

Sidecar requires Mac with Skylake processors and later models, and iPad models that support Apple Pencil.

Sidecar-supported Mac models:

• MacBook introduced in 2016 or later

• MacBook Air introduced in 2018 or later

• MacBook Pro introduced in 2016 or later

• Mac mini introduced in 2018 or later

• iMac introduced in late 2015 or later

• iMac Pro introduced in 2017 or later

• Mac Pro introduced in 2019

Sidecar-supported iPad models:

• 12.9-inch iPad Pro

• 11-inch iPad Pro

• 10.5-inch iPad Pro

• 9.7-inch iPad Pro

• iPad (6th generation or later)

• iPad mini (5th generation)

• iPad Air (3rd generation)"