Step 1: You will need...
Some PVA glue
Half a ping-pong (table tannis) ball
Some scissors (preferably small ones)
Some thin card with the template printed on
And a marble that fits inside the ping-pong ball.
Step 2: Cutting...
You will end up with two, pritty much identical circles.
Step 3: More cutting...
Cut two of the lines that are going in towards the center of the circle. Cut two that are next to each other.
You can now fold down this 'tab' that you have created. This will help keep the circles from moving in relation to each other.
Finally, cut the rest of the straight lines.
Step 4: Shaping...
Push the card into the ping-pong ball and smooth it out with your fingers.
Use the marble to help achieve the hemisphere shape.
Remove the card and do the same with the other template.
Step 5: Glueing!
Now draw lines from the centre to the edge of the template with glue. Do this on each 'tab'.
Step 6: Squishing
Now, take the other template and push it into the ball, so the two templates are both inside the ball, with a layer of glue between them.
Squish the templates into the sides of the ball, to get them both to stick to each other and to become the right shape.
It doesn't matter if glue spurts out, just smear it on the top edge of the hemisphere-it will make the hemisphere stronger.
Step 7: Finishing up
Finally, after everything is finished, just remove the hemisphere from the ball and sit it down on a desk to dry. I leave mine overnight.
Then, once the shell is dry, place it back in the ping-pong ball, and cut around the top of the hemisphere, using the ping-pong ball as a guide.
Step 8: Final thoughts
Here is the template i used for my hemispheres.