A 3D model of a human is useful for a myriad of reasons: an art project, a study of the human anatomy or even a piece of decoration. The most important thing to keep in mind is dimension. It's easy to make a human look unrealistic if you don't get the dimensions right.
Measure out the features of your body; for example, the size of your head in relation to your height or the length of your arm in relation to your height.
Use these measurements as the dimensions for your 3D human model.
Decide on a scale. If you want your model to be life-sized, keep your measurements as they are. If you want the model to be small, divide the measurements.
Mold the clay to your dimensions.
Keep water near you to wet the clay if it starts to harden. Water also makes a good adhesive for wet clay.
After deciding on a scale, divide your dimensions by a constant number to make sure that the 3D model is proportionate.