These x-es are individual cross stitches, but the 100 stitches book only demonstrates doing the cross stitch in a line. The following pictures are my step by step attempt at the cross stitch in a line. I am starting with six strands of floss - dmc color #907, using a #24 chenille needle on 100% cotton quilting weight fabric.
I will be working this stitch from right to left. First come up through the fabric at what will be the bottom right of what will be the first "x":
Now go back down through the fabric at what will be the top left of the next "x":
Pull the floss through to form the next stitch:
Continue on in this fashion for as long as you want your stitching line. When you reach the end, it is time to stitch back to the beginning to complete all the x-es! To do this, first come up through the fabric at the bottom LEFT of the end of your stitching line:
then cross over that simple stitch and bring your needle back into the fabric at the top right
Pull the floss through and this will form an x:
Next, bring your needle up at the bottom left (right next to the bottom right of the first completed x. Then back down through at the next top right: