To sign turtle, make a fist with your strong hand, with your thumb over the top of your index finger. Put your non-dominant hand over the top of your dominant hand's fist, so the dominant hand's thumb is poking out. Wiggle the thumb up and down like a little turtle's head poking out of the shell.
Do a little bit of lumbering around on all fours like a turtle as you teach the sign, to make it more memorable for your baby.
We use all the animal sign flash cards together to make a game out of identifying the animals.