This is the first stitch to master when you take up cross stitch.
On most linen and even-weave fabrics, work your stitches over two threads, as shown in the diagrams. For Aida cloth, each stitch fills one square. You also can work cross-stitches in the reverse direction. Just remember to embroider the stitches uniformly -- that is, always work the top half of the stitch in the same direction.