This is the first stitch to master when you take up cross stitch.
For horizontal rows, stitch the first diagonal part of each stitch in the entire row. Then, work back across the row, completing the second diagonal part of each 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.