well, first, this is an easier image to use (http://qutzalcoatl.tumblr.com/post/69431292164/there-must-be-more-to-life-than-this-on-flickr)
second, i’m very bad at expleaning stuff ,that’s why this took me a long time but i hope you understand it this time .
and there are two options for the squares, you can use the layer mask
inverse the color to black and use the rectangular marquee tool
and paint the selections (aka the squares) in white (all of this in the layer mask !)
or you can just paint all the squares (also with the resctangular marquee tool) and then create a clip mask
ooooooooh like two months ago ( ? )