Take your measurements right. Here's how you do it:

Bust:

Place the measurement tape on top of the widest part of your breast and measure all the way around.

Waist:

Taken on the skinniest part of the stomach.

Hips:

Put the measurement tape on top of the hip bones and measure the circumference.

Bottom:

Taken on the widest part of the bottom.

Thighs:

Taken at the middle of the thighs.

Ankle:

Taken right on top of the ankle bones.

Inner leg lenght:

Measure from the crotch to the ankle, or if you want the leggings to curl up a bit at the bottom of the leg; measure to the floor. (This measurement is a bit hard to take on yourself, so if you have someone that could help that would be good)

Arm lenght:

Taken from the armpit to the wrist.

All measurements should be taken while standing up straight. Preferably by someone else than yourself. And think about just letting the measurement tape rest on the body, don't pull too tight.