Keeping your clubface square during the swing will help you improve your direction. Where the clubface is pointed at impact is approximately 75% of the direction the ball goes. The other 25% is where you are swinging the club just after impact in the beginning of the follow through. A golfer who hits the ball way to the right or left definitely has an issue with where their clubface is facing at impact.
There is a simple drill that will help you develop the feel of rotating your arms the correct amount and keeping your clubface square through impact.
If you start in the setup position and swing back to where the club is parallel with the ground, the clubface should be pointed up to the sky and slightly turned in. Swing to this position in slow motion and then stop and check where the clubhead is pointed.
Next, swing down to your impact position and check if the clubface is pointed straight or square. Pausing for a couple of seconds to check where the clubface is pointed at these positions will give you the feeling of the arms rotating the correct amount. You are getting the feedback if you are keeping it square or not so either way you develop the feeling of rotating the arms correctly through impact.
Then swing to the position in the follow through where the club is parallel with the ground. The clubhead should be pointed to the sky or just slightly turned in like the picture below demonstrates. This position in the follow through should mirror or match the same position in the backswing.
You want to do this drill and swing to these three positions without looking while you swing the club there. Then, stop to check where the club face is pointed. Eventually, by practicing this way, you will develop the feel of keeping the club face square through impact and feeling how much your arms should be rotating.
The forearms are mainly what controls the clubface angle. If you learn to keep the clubface square through the impact area, most likely it will stay pretty square the rest of the swing. When I work on this clubface square drill with my students, 90% of the time it greatly improves their direction problems and helps to straighten out the golf shots.