How to do Abdominal bracing
To correctly brace, you should attempt to pull your navel back in toward your spine. This action primarily recruits transverse abdominis You should be able to breathe while doing this, you should NOT hold your breath. Take a deep breathe in, allowing your rib cage to expand as much as you can.
As you exhale, tighten your belly muscles while drawing your belly button in towards your spine. Just imagine a string attached to the inside of your belly button, and have that string pull your belly button back towards your spine while NOT affecting your breathing. Hold the position for 10 seconds and then relax.