The pattern will tell you which measurements you need. Make sure that the person is standing straight and that you are reading the tape measure properly.
First, you will need a measuring tape. Measure the bust from armpit to armpit, then measure the length of the body from neckto the desired length of the dress. It's also important to get a hip measurement so you'll know how wide to make the dress.
To measure your child, start by measuring loosely around her chest and then around her waist. For length, measure from shoulder to knee, or mid thigh, or ankle.