First, fill the screen black
Add a new layer and draw in white all over it. Duplicate it twice (layer-duplicate) then click the bottom (1 up from the black one) and go to Filters-
Blur-linear blur. St it to X 100% Y0% quality 7 and continues. Duplicate it to make it brighter.
go to the next layer up and do linear blur with the same settings but X0% and Y100%. Duplicate that one too.