Although you can perform a plie from any of the five positions, it is easiest to learn from first position. First position is when your heels are together and your toes are turned outward. Keep your hips tucked under so your bottom doesn't stick out and keep your back straight and shoulders back. One hand should be on the barre or wall and the other should be in first position. First position arm is down, rounded from shoulder to fingertips, with the elbow slightly out. Your hand should be near the pelvis and fingers should be curled slightly.
For the demi-plie, bend your knees slightly. Again, make sure your bottom is tucked in and not sticking out. As you bend your knees, your arm should keep the same curve, but lift it to chest height.
Press your heels into the floor; do not let them lift up. Keep your knees over your toes.
Using your core and thighs while pushing your heels into the ground, pull yourself back into first position. Bring your arm out to the side, look slightly at the arm and then bring it back to first position and return your head to the middle.
For the grande-plie from first position, your heels will come off the ground. Bend your knees deeply, keeping your knees in line with your toes and your hips pulled under to prevent your bottom from sticking out. The space between your hips, knees and heels will create a diamond shape. Arms are the same as with the demi-plie