About five minutes, once. Nothing here needs a computer.
Most of this is installing Shizuku. It is the only awkward part, it is a one-time job, and the reason it is needed is explained below — it is not busywork.
While a foldable is open, Android switches the cover screen off completely and hides it from every app. No permission, setting or Good Lock module changes that.
There is a way through: your phone already has a mode that powers both panels at once — it is what the camera's dual preview uses. Cover Screen Trackpad asks the system to switch into it, and asking requires a privilege ordinary apps do not have. Shizuku is a free, open-source app that grants exactly that privilege, and nothing else.
Shizuku is widely used and does not need root. It is a separate app with its own developer and its own privacy policy.
Use the project's own site rather than the Play Store. Play reports Shizuku as incompatible on many phones where it works perfectly well — that is a restriction on the listing, not on your device.
Settings → Developer options → Wireless debugging. If Developer options is not there, go to Settings → About phone → Software information and tap Build number seven times.
In Shizuku's settings, turn on Start on boot. Without it, Shizuku stops when you restart the phone and the trackpad will not work until you start it again — this is the single most common cause of "it worked yesterday".
Shizuku almost certainly stopped when the phone restarted. Open Shizuku and start it, then turn on Start on boot in its settings so it does not happen again. Cover Screen Trackpad's main screen detects this and shows a button that takes you straight there.
Check the status line at the top of the app — it says what is missing. The usual causes are that the phone is not fully open, or that Shizuku is not running.
The pointer is also hidden on purpose while the settings screen is full screen: your finger is placing the active zone rather than driving the pointer. Leave the settings screen and it comes back. If you want to change settings and use the trackpad at the same time, put the app in split screen or a pop-up window.
If you ever installed an earlier build of this app under a different name, two copies of its accessibility service may be running and competing. Uninstall the old one — anything called FlexPad — and restart the phone.
Rare, and it means a session ended without cleaning up. Open and close the phone once with Cover Screen Trackpad running, and it resets itself.
Barely. The cover screen is pure black and the panel is OLED, so almost no pixels are lit — measured on a Flip 6, a running session costs about 4 mA more than having the main screen on, which is inside the noise of ordinary use.
If you want to be thorough, the idle fade-out setting ends a session automatically when you stop using it, and pointer memory brings the pointer back where you left it.