Here are the steps required for exporting a textured object from Blender3D to Unity 3D.

1. Select the cube you want to texture.
2. Name the selected object “Box” (double-click on the cube in the scene/hierarchy view in the upper right and type in a new name for it)
3. Now is a good time to save your Blender file, give it a name like “box.blend” and keep all your files for this model in the same folder (to see these files in Unity, keep them all somewhere under “Assets”, e.g. “Assets/VIEN/Props”)
4. Switch to Edit Mode (The mode selector currently shows “Object Mode” in the upper left)
5. Switch to Edge Select Mode (Three buttons immediately to the left of the mode selector: vertex, edges and polys)
6. Multi-select (hold SHIFT) the “seam” Edges of the Box (for a fold-out cube)
7. Edges→Mark Seam (Menu above the view port)
8. Select the UV Editing tab at the top of the screen.
9. Select your object again in the 3D view (press “a” to select the whole object)
10. If a UV image appears in the UV Editor window on the left, showing how all sides of the cube fit onto the image, then you can skip the next step (the UV coordinates already exist, so no need to unwrap again, unless you want to try)
11. Either window: UV→Unwrap (you should now see the fold-out texture from the cube appear in the left UV Editor window)
12. UV Editor Window: UV→Export UV Layout.. (you should save it in the same folder as your 3D model)

• And now you have a texture which you can edit in your favorite paint program (like GIMP, which is free!). Once you're done painting it, you can import it back into Blender like this:

1. UV Editor Window: Image→Open… (pick your file here and you can preview it in the UV Editor)
2. To apply the texture to the cube object, you must now create a Material for the cube, and assign the image to its color. To do that: Go back to Layout tab at the top of the screen (only the 3D object being shown). Select the Material property tab for the object in the property inspector on the right (the Material property looks like a red planet towards the bottom). If no Material is shown there, press the drop down to the left of the New button and pick 0 Material, which is the default white material. Now add the texture to it by clicking on the circle to the right of the color shown for the Base Color property and pick Image Texture. Then in the new image texture properties that appear, select the texture file, either using the picture icon drop down (where you can pick images already imported into the project, such as through the UV Editor Window) or by pressing Open and pick the texture file there. The texture has now been applied to the cube, but it may not show yet in the 3D view, since by default textures are not rendered.
3. To see the texture on the cube in the 3D view, choose Look Dev or Rendered from the Viewport Shading options in the upper right corner of the view port

1. Unity can automatically import .blend files, so if you have saved your Blender scene inside a folder that is anywhere inside your Unity project folder, then you can simply navigate to that file on the Unity side and drag it directly into your Unity scene!
2. Every time you save in Blender, Unity will re-import the object.