In order to make a swimsuit pattern, knowledge of how to draft basic clothing patterns is necessary. A swimsuit pattern, like most clothing patterns, is merely the transformation of a basic pattern with the added challenge of calculating the reverse ease of the material which is being used. Sizing for a swimsuit will not match that of a basic pattern. The pattern pieces are cut smaller based on the estimated stretch factor of that particular fabric and will only be useful for fabrics of similar elastane content.
Draw a picture of the swimsuit you would like to design. It is not necessary to have a complete fashion figure to design a swimsuit. You may just use a torso template.
Choose the fabric you will be using. You must have the fabric prior to creating the pattern to determine the stretch. Pay attention to how the fabric stretches. Check whether the fabric only stretches in one direction or in horizontal, vertical and on the bias. Bias is basically the diagonal stretch of the fabric.
Calculate the reverse ease by measuring the stretch for each of the directions. One way to do this is to wrap the fabric around your hips and pull it snug to where you feel it is comfortable. Mark the point where one end of the fabric meets the fabric on the other side. Now release the fabric and measure the length of the fabric unstretched. Subtract the unstretched measurement from your hip measurement. If the fabric does not fit around your hips use some other point of reference, for example your thigh or calf.
Draft the pattern subtracting the stretch factor ratio from the base measurements. Pay careful attention to which direction the pattern will be placed on the fabric. If the fabric only stretches in one direction it is only necessary to adjust the measurements for the corresponding calculation.