Help with Creating a Light Map

So for the holidays, I would like to have more festive lights and the like, and I really like the way they did the ones on the tree, wreath, and garland curtain in Seasons. Through some tutorials on here and the like I think I have everything figured out in S4S in terms of the modding, but I can't figure out how it is you make a light map itself.

I asked someone last year and they told me that I needed to create an alpha layer and copy the blue layer(?) onto the alpha layer, but I'm very confused with that. I've been using photo editing software for fifteen years, but I don't know anything about alpha layers.

Basically, I want to know how to make a light map with this texture, where the bright parts are where the bulbs are, of course: