Another way to look at this is to think of elongating the breath. Relax into it slow down the movement and think of the breath coordinated with the movements this will set the pace of the movement as well. As the others have mentioned don’t focus too much on the breathing. Sometimes what happens is we focus so much on the end result that we take into much or do too much.
Take it a step at a time, to much too soon will complicate the process and diminish the overall effects. When you become more comfortable with the pattern and the process you can then add the thought of reverse abdominal with the breath work then eventually bring in the actual process and so on. Think of bringing in the lower abdomen in the last 75% to 80% of the breath extent. In other words breath in normally, elongate the breath, visualize and then just at the end of the breath bring in the lower abdomen gently and slowly for the last bit of air. This will help you adjust to that process and eventually it will begin to feel natural and you will be able to start the movement of lower abdomen sooner. Remember the abdomen does not move much too much and to much effort can shift unnecessary pressure to the upper chest area. Remember everything is gentle and relaxing if you cross the threshold into discomfort then listen to your body and back off a little. If you take this approach in stages it will be much easier to combine them into one process later as you progress because you will have a better foundation to grow from. Good luck, I hope this helps. Wishing you well. 
