As with all core work ensure a neutral/lordotic low back position. This should be a position that is pain free and for the bird dog you should be able to balance a tennis ball on the small of your back without it rolling off through the entire movement. If you can do this you’ll know your spine hasn’t moved and only the ball and socket joints of the shoulder and hips have. Brace your core, slowly elevate your right arm and left leg back until they are parallel to the ground. Hold it for 5 seconds. Return it slowly and controlled to the start position and repeat with your opposite arm and leg. Aim for 10 repetitions on each side with a 5 second hold each. If you are too fatigued and you cannot hold form then simply hold the position for less time. Once you can do 10 repetitions per side for a 10 second hold each you are ready to advance to the next move.