The most important factor determining where the ball will go on any given shot is the club face position at impact.
If the club face is open at the point of impact the ball will fly, at least initially, right of the target (for a right handed golfer). The same is true of a closed club face position at the point of impact. Because the club face is the most important factor in deciding where the ball will finish, we need to raise our eyes up the club shaft and focus on the hands which are the only part of the body in contact with the club throughout the swing (we hope!).
The way the hands rotate during the back swing and through swing are vital to how the club face will look at impact. If the hands rotate too little on the back swing, the club face will likely be returned closed to the target line at impact. Too much rotation and the club face will likely be open. It is the same story in the down swing but switched around, too little hand rotation in the down swing will lead to blocked and sliced shots, too much hand rotation will lead to pulls and hooks.
This might sound overly complicated but it can be quite easily understood and controlled by comparing your swing to a few easy checkpoints. All of the checkpoints relate to right handed players employing a neutral grip.
Hand rotation swing checkpoints
This is the correct hand rotation throughout the swing. It will be almost impossible to check these positions in real time whilst hitting the ball so some use of a video camera or mirrors will be involved.
If these are not available then complete a number of slow motion swings until the sequence is understood and repeatable.