Two things...The board..and paddling technique.
Some really short wave boards will 'yaw'..or want to turn with every stroke..no matter how good you are.
Most boards can be controlled with a better technique..You need to paddle deep..close to the board..and with the paddle shaft almost rubbing the rail.
The main thing is to have your left hand way over the rail on the right side of the board when paddling on the right side..and visa-versa.
This keeps the paddle shaft more vertical..Any angle in the shaft when looking straight on..will make the board turn more.
Practice on flat water and you will soon have it down pat...Dipping or holding down the rail on the same side that you are paddling on will also help.
Hope this helps.....Anyone else want to add their thoughts about this?
DJ
also...I took this vid of myself a while ago..It sorta shows what I mean..