It depends how limber your calves are to start with. You can lean against a wall, put your legs behind you and balance on the ball of the foot on the side you want to stretch, but be careful how far your legs are out behing you if you have any issues with your back. If you sit with both legs out in front of you like you were going to stretch your hamstrings, point your toes and lean as far forward as you can; this will stretch your shins a little. I like to balance the ball of my foot on the edge of a treadmill or some elevated surface and (balancing myself with any nearby surface) stretch my calves out one by one. A marathon runner also told me that writing the alphabet in the air with your big toe a few times a day was helpful for this type of thing.
P.S. Your "sharktank" is exactly the same concept as our renduri, only we don't roll unless it's TKD sparring (olympic rules and therefore full contact).
Noted. I'll be stretching them daily, thanks!