It is assumed that you can insert new layer, and can work with rectangular selections.
See following logical diagram carefully. When you make something qubic you have its 3 sides viewable. So logically we create only 3 layers to created wrapper of our product box, see image:

Follow steps.
Create new image, I used 300x300 pixels.
There might be a layer already, delete it.
Insert 4 layers, and name them from top to bottom, as
1. Box
2. Top
3. Front
4. Side
Select layer number 3 named Front. and select 2/3 of its total width (means 200 pixels), as shown here

Go through menus (within image window) Layer > Crop Layer
Now select layer named Side (notice that selection is still active) press Ctrl+I to invert the selection, or simply go though menu Select > Invert, and repeat the layer crop by going through menu Layer > Crop Layer.
Now select layer named Top, create a selection on it 1/3 of actual height (means 100 pixels tall), and Crop it same way, btw it does not matter if you have good guess and few pixels are wrong. See here:

Go through menus Select > None to remove any selection.
Now draw whatever needs to be drawn on the front, side and top view of box, keep in mind the logical picture shown in start of tutorial.
Upto here, you have completed 75% of the task.
Now select the layer named Box which is completely empty and will be used to draw box on, go through menus (within in image window), Filters > Map > Map Object. Here it is a bit tricky. You can see a new window appeared (if not then get new gimp). Note encircled options in screenshot below:

Now goto Box Tab and select layers one by one
For Front , select layer named Front
for top, select Top
and for left and right select Side.
Now go to Tab Orientation and play with Y under rotation, and hit OK,
your box is ready.
I will add some more about making its shadow.

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