Each pixel makes an x.

To sew that you begin from the inside/back of the shirt and poke the needle up through the bottom left corner of the pixel you are working on. Then sew through the top right corner. You now pull the needle through from the inside of the shirt to the bottom right corner. Finally you sew through the top left corner of the pixel and that square is now complete!

If this is your first time cross-stitching, practice on some scrap fabric first until your stitches look consistent.