A push slice golf shot is one where the ball starts to the right of the intended target and then curves even more to the right during it's flight, finishing further right of the target.
It is caused by swinging the golf club so that the club head travels on an in to out swing path with the club face open. To explain exactly what that means, place an alignment pole on to the ground to represent the target line, or the line that we want to hit the ball down. The far side of this line is known as the outside and the side that you are standing on is known as the inside.
A push slice ball flight happens when you swing the club head from the inside of the target line to the outside, or far side of the line, with the club face aiming right of this direction of travel. The ball will start to fly right of the target because of where the club face is aiming, which is to the right of the target, but it will then curve further right in the air due to the difference between the club face angle and the direction of travel of the club head. Any difference between where the club face is aiming and the direction of the club head’s swing path produces backspin that has a tilted axis. This tilted axis backspin results in a curved ball flight when the ball is in the air and as a result of the club face aiming right of the swing path, the curvature is to the right so the ball finishes further right of the target.
In order to hit a straight shot rather than a push slice, we need to swing the club head along the target line with the club face aiming along the target line and this will produce a straight ball flight. A great drill to encourage you to do this is as follows. Place an alignment pole or club on the floor to represent the target line. Now take six golf balls. Place three of them on the outside of the target line and three on the inside. Of the set of three, one needs to be placed two to three inches directly opposite the ball on both the outside and inside of the target line. Then place two more balls, one on the outside and one on the inside, one inch to the left of this and the final two, one inch to the right. This will create a channel for you to swing the club head through.
Make some practice swings and work on striking the club head into the ground in the centre of the channel, without hitting any of the additional six guide balls. As you do this, keep the club face aiming along the target line at the target, until the club head has swung through the guidance channel. Once you feel more confident and comfortable with this and you find that you are not hitting any of the guidance balls, start to hit balls again working on achieving swinging the club head through the centre of the channel (along the target line) with the face aiming at the target as you do this.
With some hard work on getting this movement correct, you will very shortly be hitting much straighter and longer golf shots and getting back to really enjoying your game.