Hiii, I would love to fit the instructions here in the description so you can read it here instantly without having to watch the vid but it’s hard to describe. Here I go anyways:
To make mask
Tap and hold on the theatre mask-looking thing on the gizmo and the brush over the mesh.
To invert mask
Hold on to the mask button and tap outside the mesh, that is somewhere in the viewport
To clear the mask
Hold on to the mask button and draw a rectangle somewhere in the viewport
Video timeline
Instructions: 00:00
Bonus Tip: 02:45
Check my sculpt progress: 4:33