Navasana is a fantastic pose to challenge gravity and put the core muscles to test. To avoid the lower back sinking toward the floor, engage the abdominal muscles and bandhas. This will create the stability and support required to allow the the shoulders to draw back and the chest to shine open.
Find a comfortable seat with the weight distributed evenly between the sit bones and the sacrum. Inhale. Bring the hands behind the knees and lift both legs to eye level, parallel to the floor with the feet together. Exhale. Release the hands straight and parallel to the floor, palms either facing together or to the sky. Drishti is towards the nose.
Remain here for 5 deep breaths.
Imagine a magnet between the knees and the chest, pulling them together and keeping the abdomen engaged.
Feel free to keep the hands behind the knees for support. Bending the knees will make this pose easier, raising the arms over the head will make it more challenging.