First you glue these two parts together with the floor and lid.
Next, the shelves.
To ensure that the shelf is straight, you should put the bottom and lid as long as the glue is damp and stick on the back wall.
As soon as the glue is dry, you can paint the shelf inside. This better goes as long as the front is not glued on.
Now only stick and paint the front. Then the shelf can already be decorated.