I need help.

I would like to dim LEDS of a Christmas tree with a potentiometer. Here is my electric diagram. (Please note I'm a beginner in electronics.)

I don't know how to choose a good value for the potentiometer. Is my diagram 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 31V P=3.6W