The key to hitting the ball straight (without curvature) is to keep the clubface square to the swing path.
There are three places in the swing where the clubface needs to be square: at address, at the end of the backswing,* and at impact.
Address. Step behind the ball, looking downrange. Visualize a line connecting the target and the ball. When taking your stance, place the club on the ground so the leading edge of its sole is perpendicular to the line.
Backswing. Swing back as far as you normally do, and stop. Remain coiled, and test the position of the clubface by lowering your arms straight down from where they are, to just above hip height. Turn your head to look at the clubhead.
If the leading edge of the sole is straight up and down, it’s square.** There’s nothing more for you to do but practice the feeling of that backswing over and over to make it your own.
If the leading edge leans either forward or backward a bit, this is the result of what feels normal to you, and you now have some work to do to replace your customary backswing feeling with one that keeps the clubface square.
The solution is in your hands and forearms.
Let’s say your clubface was closed. Swing back and rotate your forearms slightly clockwise as you do. Not too much. Just a bit. When your backswing is completed, perform the test. You can see right away if you got it right, if you went too far, or not far enough.
If you’re corrcting an open clubface, rotate your forearms slightly counter-clockwise.
Keep working until you get it right. Then remember what that backswing feels like and practice it over and over.
Identify a key feeling, too. What I have found is that when I am keeping the clubface square all the way back, the feeling of my hands on the handle that I have at address does not change. The feeling that clicks for you might be different.
Impact. You’ll need to hit golf balls to figure this one out. If your shots fly to where you are aimed, go out and break 80.
If they curve,*** or if they fly straight, but to the right (push) or left (pull) of where you are aimed, get a lesson to find out what you need to correct. Those errors can’t be fixed by reading a blog post.
* It is possible to play good golf from an open position or a closed position at the end of the backswing, but it so much easier to play from square.
** If you try this test when you have taken the club back only halfway, to hip height, a square leading edge will be leaning forward a bit. Performing this test from a different position gives you a different result.
*** To an unplayable degree.