Look at the front and back of your step-down converter and note where the trimpot connects, as well as the value of the trimpot. In this cake it was 50kOhm

Remove the potentiometer either by desoldering it properly, or lacking tools, by bending it back and forth until it snaps off. Now solder the potentiometer in place of the trimpot. The order of the wires is the same as the trimpot was: if in doubt it probably goes the way you think it does.

Install in a case. I used an RC LiPo battery clip as input as I have a spare battery lying around I don't use, it's physically impossible to plug in in the wrong polarity, and it's easy to find replacements.

As you might expect, turning the potentiometer sets the output voltage.