How do you get an hourglass from this shape?

Rep-4 L-tromino

In fact, it’s easy. You simply divide the shape into four identical copies of itself, discard one copy, and repeat the process with each of the sub-copies:

Constructing an hourglass (animated)

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Hourglass (static)

Here are some more posts about what I call the hourglass fractal:

• The Hourglass Fractal at Overlord of the Über-feral