This page is for the harder version of the problem: not a brow that has thinned but a brow where the tail has largely gone, the arch is down to a scattering of hairs, or there is very little left to work between. The technique for a thin brow does not scale up — you are no longer filling gaps between hairs, you are drawing a brow that mostly is not there, which is a different task.
The key principle is counterintuitive and it is the whole page: leave gaps. A real brow is not a continuous mass of hair; it is individual hairs with skin visible between them, and the density varies along its length. When you draw a brow with no gaps, the eye reads it as a solid shape — and a solid shape above the eye is the thing that looks drawn on, regardless of how good the shade is.
So on a very sparse brow you draw fewer marks than feels sufficient, spaced apart, varying in length and angle, and you resist the urge to connect them. Then you add body with a tinted fiber gel, which deposits tiny fibers that catch light the way hair does and fills the visual space between your drawn marks without making them continuous.
The second principle is structure before density. On a brow with almost nothing left, decide where the brow goes before you decide how dark it is: where it starts, where the arch sits, where the tail ends. Those three points determine whether a drawn brow looks like a brow, and a well-placed sparse brow reads far better than a dense one in the wrong place.
A note on the thing that is often suggested at this point: semi-permanent brow tattooing and microblading are real options that many people are happy with, and they are not something this site can advise on. They are a cosmetic procedure with practitioner-dependent results, and the right people to ask are practitioners and, where relevant, a doctor.
