I add shading but it still looks completely flat

Why this happens

Flat shading usually means the shadow is only slightly darker than the base, or the light source isn't consistent. The fix is a two-layer system that works with how light actually behaves.

FIX IT NOW

1. Create a new layer above your base color. Tap N on the layer → change blend mode to Multiply
2. Pick a dark, slightly warm brown — not pure black. Set brush opacity to 40–50%
3. Paint your shadows on this Multiply layer. Because of the blend mode, the color underneath shows through and tints the shadow naturally — it looks integrated instead of painted on top
4. For highlights: create another new layer. Set it to Add or Screen blend mode. Pick a slightly cool light color. Paint only the brightest hotspots — not the whole lit area, just the peak highlights
5. The two-layer method (Multiply for shadows + Add for highlights) creates the illusion of volume because you're working with how light physically works instead of just painting darker and lighter versions of the same color