I have students who ask all the time what training aids they should buy to help improve their game; one of my favorite training aids is a cardboard box. There are so many ways a cardboard box can be used, but one of my favorite ways to use a box is as an obstacle for swing path drills. It makes a great visual and gives you immediate feedback if you hit it. but does not damage your club. If you ever watch Brooks Koepka warm up, you will likely see him using a box for some of these easy drills.
The most common mistake I see in students is a too out-to-in path, so in this article I will be demonstrating ways to get rid of the excessive out-to-in path. Align the box for the swing path you want to practice (outside the target line if you are out-to-in too much). Once you have your box in place, as I have demonstrated (box outside the target line at a 45-degree angle), rehearse some swings with your clubhead next to the box, making practice swings swinging toward third base (the right corner of the box). From there, have your clubface square to the box at impact, as I have shown in the picture against the blue alignment stick. It is important if you are wanting to hit a draw or lessen your slice for your club path to be in-to-out and your clubface to be square at impact. If your clubface stays at the same angle as your path at impact in-to-out in this drill your clubface would be open, making you hit the ball straight right.
Once you have rehearsed some swings, swinging more in-to-out and having a square clubface at impact. Place a ball on the inside corner of the box only a couple inches away from the box and hit shots missing the box. If you hit the box placed outside your target line at a 45-degree angle, you are swinging out-to-in too much and will send the box spinning. You will also see a big left-to-right ball flight with your ball (if your golf ball is home plate in baseball, you are swinging toward third base and need to swing toward first base to make the ball go straighter).
If you do not have a box, another training aid I like to use is an empty water bottle. For this drill, place an empty water bottle outside the target line six inches left of the ball and one inside the target line six inches right of the golf ball. This will give you a visual of having an in-to-out path between the water bottles, as shown in the picture. Make some small swings visualizing an in-to-out path, as the yellow arc below shows, and then hit some full-swing shots after you have rehearsed the small swings visualizing an in-to-out path.