Hold the top thumbscrew with one hand and loosen the bottom lock nut with your other hand. You only need to loosen it a half turn so the entire outer roller assembly can move laterally. Next, turn the small round thumbscrew on the outside of the carriage clockwise to tighten the preload and counterclockwise to loosen it. After you make an adjustment to the preload tighten the roller assembly and test your slide. We normally recommend to try and twist the carriage on the rails. If there is too much play you will want to tighten it up in small increments until you get your desired preload. Make sure to adjust the preload on both sides of the carriage equally.
Note: In most cases you’ll never have to adjust your preload, it’s mainly an added feature for increased control of friction and tension.