Go to Edit > Preferences then to the Add-ons tab. Click Install.

Choose the downloaded zip and click Install Add-on.

After the installation is complete it will show up in the add-on list. Enable the add-on by ticking the checkbox.

Open the Q-Decal tab in the 3D Viewport N-Panel. Select a mesh object with a material, pick a decal from the Decals panel and click Add Decal Layer.

The decal is placed with a modal grab: snapping is temporarily enabled with Face snap and align rotation, so the decal follows the surface under the cursor. Click to confirm, or press ESC / right-click to cancel and undo everything.

Hold Shift while adding to apply the decal to all materials of the selected objects, or Ctrl for all materials of the active object. Adding works from an empty too, as long as its parent is a mesh.

Each decal is driven by a controller empty (stored in a QDecals collection) that is parented to the object. Move, rotate and scale the controller to place the decal; the layer's Mapping panel offers the same controls numerically.

Custom: set the custom library path in the preferences. The Load Decal Layer button (refresh icon) opens a file browser for PNG, JPG, BMP, TIFF, TGA or EXR images. Loaded files are copied into the library folder so they appear in the preview browser for reuse.

When a folder of textures is loaded, maps are auto-detected by filename identifiers (base color, AO, metallic, roughness, normal, opacity/alpha, displacement) and connected to the matching layer slots with the correct color space. A single image is used as the base color.

Megascans: set the path to Megascans Library/Downloaded/ (the folder must contain assetsData.json). Browse decals by category and subcategory, choose a size (1K/2K/4K) in the preferences, and add them like custom decals. Albedo, AO, metallic, roughness, normal, opacity, translucency and displacement maps are loaded automatically.

Every material with decals has a Layers list. Reorder layers with the arrows (the node chain updates live), duplicate a layer with its controller and settings, or remove it. The eye icon toggles visibility of all layers; the panel header icon toggles every decal in the scene.

Each layer exposes Opacity and Emission strength, plus a controller picker. The Maps sub-panel shows the layer's textures with blend type, mix factor, tint color and normal strength for the base color.

Wear: enable per-layer procedural wear that erodes the decal based on the mesh. Amount, Scale, Detail, Contrast and Offset are exposed directly in the panel, and the wear node group can be edited in a separate window via the node-tree icon.

Use Duplicate Object with Decals in the material panel to copy the object with unique decal layers, node groups and controllers, so the copy can be edited independently.

Libraries: Megascans library path and import size, custom library path, and the filename identifiers used to auto-detect each map type.

Controller: display type (axes, arrows, circle, cube, sphere, image) and display size of the controller empties.

Preview: icon scale, popup scale and labels for the decal preview browser.