First you should think about where the switch finds its place, keep in mind that it also has enough space inside. With the enclosed template, you draw the opening and then expose it with a sharp cutter and put the switch in his place.
Take the two red cables from the LEDs and twist the exposed (metal), at the end of the cable together with any cable from the switch (here the color varies).
Then, in the same way, connect the two black LED cables with the black cable of the battery box.
Now you only have two cables left together.
Now you put a piece of shrink hose over the twisted cables because the open (metal) places are not allowed to touch. Before you heat the shrink tube with a lighter (until it has adapted to the cable), you should test whether the LED shines.
For the LED test you first place batteries and if the light does not light up directly then you overturn the switch. If there is still no light, take a photo of your cabling and send me this for error analysis to mailto: firstname.lastname@example.org