Let me talk you through the sample you're looking at from the bottom up:
I did a base chain of 23 stitches. I then did a row of 21 treble (double) crochets, turning chain counting as a treble (double).
Then I thought I'd alternate two treble crochets then two crossed stitches, then two trebles again, and on in that pattern just so I could get used to it. In the following row I crossed all the pairs of stitches leaving the turning chain on one end of the row and one treble on the other end. It was fun and easy. This is the first new stitch I'm learning in the Stitch Library section of my partwork.
I practiced this using the practice yarn supplied, but I am now confident enough to make a square entirely of crossed treble crochet. I'm going to make a piece called "learning curve" which will contain every new stitch or pattern of stitches I learn. It will be quite some time before I finish as I have every new stitch given to me in the partwork until the partwork is over, and since I don't know how many squares I will end up with, I can't really make anything with them til I'm finished.