There are two pieces that should definitely be placed last or second to last. Try to identify them.

They are the two flat (2D) pieces. Set them aside for now and start by combining the 3D pieces. To begin with, simply connect any two of them.

Take these two pieces. They belong on the bottom layer of the cube.

You should end up with a shape like this:There is one more piece on the bottom layer.

Now it's time to bring the two flat (2D) pieces back into the puzzle :)Placing the last 3D piece first is still a perfectly good approach. If you haven't put the cube into the frame yet, continue with the 3D piece—don't be afraid to rotate your assembly freely in your hands.If you've already placed it into the frame, look for the flat (2D) piece that completes the second layer.At this point, it's simply a matter of which approach feels more comfortable for you.

Лишилося всього дві, ви впораєтеся ;)