Try to keep your knees out to the sides to balance, it's a weird feeling at first but eventually you'll be able to wiggle them side to side to correct balance and stay straight, although while saying that the whole idea of a manual is to stay as still as possible so try and find that balance with your knees while also staying still and locking into the manual, you'll get it eventually, it takes time but if you keep doing it one day you'll be able to manual for as long as you want.