WAIST
Wrap the measuring tape around your torso at the smallest part of your waist. Typically this is an inch or so above your belly button and is also known as the natural waistline.

HIPS
Wrap the measuring tape around the widest part of the seat.

Fitted through the hip and thigh, then opens wide from the knee. Cut to shape the glutes rather than hang loose there. Take your usual size, or size up for a more relaxed fit through the leg.

LENGTH
The leg runs long by design. These are the heights each size falls full length on, just meeting the shoe:
XS, 106.5 cm leg → up to about 167 cm
S, 107.5 cm leg → up to about 169 cm
M, 108.5 cm leg → up to about 171 cm
L, 109.5 cm leg → up to about 173 cm
XL, 110.5 cm leg → up to about 175 cm

Shorter than that and they drape over the feet, which is the look in the photos. Taller and they sit closer to the ankle.

The length grades about 1 cm per size, so sizing up adds room through the leg and only a little extra length.

The foldover waistband gives you more room again: worn up it sits higher on the waist, worn folded it sits low on the hip.

Garment measurements in cm, if you'd rather compare against something you already own (circumference and unstretched; the waistband stretches well past the figure shown):
XS: length 106.5, waist 64, hip 74
S: length 107.5, waist 68, hip 78
M: length 108.5, waist 72, hip 82
L: length 109.5, waist 76, hip 86
XL: length 110.5, waist 80, hip 90