We're nearing the end now. The puzzle blank has been built and been shrunk a little bit so that it works in the real world. Now to shape it like the sun model and make three more. Fortunately, this is going to be very easy:
Select the puzzle blank.
From the Modifier tab, click on the Add Modifier button.
Choose the Boolean modifier:
Click on the Object dropdown and select the Icosphere object:
After a moment, the puzzle piece should effectively disappear. Where did it go? It's still there, but it's now shaped exactly like the part of the sun face it covered.
In the menu at the bottom of the 3D View, click on just the second layer button to see the piece by itself:
See? There it is—perfectly printable flat bottom and everything. Now, to make the other three pieces, continue with the following steps:
Click on the eye button in the Boolean modifier to temporarily disable it:
Duplicate (Shift + D) the puzzle piece and Rotate...