Whist this video shows a clearly defined granny square with three chains in the corners and 1 chain between the corners and the middle spaces - for crocheters with looser tension, I recommend the following:
do 2 chains in the corners and NO chains between the corners and the middle spaces.
Also - make your beginning loop only 4 chains long instead of 6 chains.

This video shows how to make the first round of a popcorn granny square

This video shows how to make the second round of a popcorn granny square

This video shows how to make the third round of a popcorn granny square

This video shows how to make the fourth round (final round) of a popcorn granny square. It is made up of groups of trebles, not popcorns!

This video shows how to make rounds 1 - 4 of a Stripey Granny Square. You can continue to make more rounds in exactly the same way - all that changes is the number of trebles on each side of each round. The corners are always 2 tr, 2 ch, 2 tr and to join end of the round to the beginning you always make a htr in the top of the ch-3.
PLEASE NOTE: In the beginning of the video it should say "make a slipknot", not make a slipstitch! Just checking if you're awake!