Lineskier10, assuming you're a guy:
Step 1:
While keeping the tape measure level, measure the circumference of your waist at
the umbilicus (i.e., belly button). Measure three times and compute the average.
Step 2:
Measure your wrist at the space between your dominant hand and your wrist bone, at the location where our wrist bends.
Step 3:
Enter your height, weight, and wrist measurements on the on-line calculator.
Link to the calculator
A multi-point caliper measurement by an expert is better (they use front of thigh, midpoint on your waist, midpoint of your back, and I think somewhere on your arm). It's a little harder to be consistent though.
Best method is a displacement tank where you get submerged in a vat of water.