Start by lying on your back with your hips and knees flexed such that your feet are flat on the floor or bed.
Next, exhale as much air as possible. This raises your diaphragm and, much like an empty stomach, allows for maximum contraction of the transversus abdominis.
Lastly, pull your navel in as close to your spine as possible. The more your navels draws in, the more the transversus abdominis is contracting.