I need help, I would like to dim LEDS of an Christmas tree with potentiometer. Here is my electric diagram (Please note I'm a beginner in electronic) I don't know how to choose the good value of the potentiometer. Does my diagram is correct? I noticed that by adding a resistance in series on the postive side of the out powering cable I manage to lower the brightness of the tree, the goal of the project is to be able to adjust the brightness with the potentiometer. Here is a picture of the power supply.

The Christmas tree