You can place your other hand on the wood, to steady it (as long you keep it well away from the blade).
Ryoba saws
The ryoba usually has a longer thinner handle which can be gripped with both hands.
When using the saw to make flush cuts, you may find it easier to grip the saw with only one hand.
When using either style of japanese/pull saw, you should try to keep your back straight and your feet shoulder-length apart.
It may sound obvious but don’t cut a line that you can’t see. Try and position yourself above and behind the cut in order to keep an eye on the direction of the saw.