The most importants things are :

- keep your feet stucked on ground, shoulder width apart, legs at 90°
- squeeze your buttocks (and only) : it's very, very, very important.
- Your shoulders must be 200% supported : very, very important.
- Take a position that relieve neck tension at maximum if your not confortable at start.
- Take a big ball, i think it's better. <75cm don't support shoudlers (i'm 5'7). 55 sucks, and i'm fit. But 65 cm is not that bad.

And breathe through the belly and don't move anything for 1 minute. Eventually do it again 15 minutes later, and wait 1 hour or 2 to did it again.

The more is better here. Realy. After some days you will start to feel your 'upper' scalenes light & weak.

If you think to have a probleme in the lower plexus, make your scapula supported too. So you may need a bigger ball.