The basic measurement is the foot length. Have the wearer stand up and measure from the back of the heel to the tip of the large toe for overall length. Measure from the heel to the tip of the small toe for foot length (the difference should be about 2 inches in adults).
If the person has a wide foot, measure around the ball of the foot, for the circumference distance to adjust the total number of stitches needed.
The cuff length should be measured from the floor up the leg to the height that the wearer prefers, 2 ½” of this is the heel flap, the remainder is the length of the cuff/leg before the heel.
Measure around the calf at this height as well, especially if the wearer has larger calves, so that the number of stitches to cast on can be determined.
Make a gauge swatch if desired, by casting on the number of stitches for 4” on the ball band on your smaller needles and knitting up about an inch of ribbing on the smaller needle. Measure them, to be sure you are working to gauge, and then multiply the number of stitches per inch knitted by the circumference of the calf. Adjust the number of stitches to be divisible by 4 as necessary. Subtract stitches for a snugger cuff, add stitches for a looser cuff.