Once downloaded, locate to Blender>Edit>Preferences>Add-ons or Extensions and install the extension by either clicking on 'Install from Disk...' or by dragging and dropping the extension onto the Blender window.

This is registered as an Extension! If you wish to uninstall the extension, go to Blender>Edit>Preferences>Extensions, find the small DOWN ARROW at the top-right corner of the extension panel. You can then Uninstall from there.

When the node does not show up immediately, go to Compositing tab, hover your mouse over the Compositing window and open the N-Panel (press 'N' or click / pull the small arrow). Locate 'Aperturia FX' tab and click on 'Restore Aperturia FX'. This will rebuild the node. You can also use it to restore any lost elements of the tool.

Shadow Mask requires the use of render passes listed below. These passes hold far better data and for best results, all passes should be enabled and connected to their distinct inputs inside the node. Also included is an output node that allows you to see your shadow mask in real time so you could adjust it exactly to your needs.

Performance: The extension computation-heavy. To speed up the workflow and ensure more pleasant experience, you should access the Aperturia FX panel and press 'Optimize'. This will change your compositing device from CPU to GPU, greatly speeding up workflow.