This potholder is designed to be worn over the hand -- or, at least, used double thickness. So, the next step is to crochet the 2 pieces together.
This will be done using a single crochet stitch. Slip stitches are also often used to crochet pieces together. In fact, just about any crochet stitch could be used. However, I selected the sc since it's sturdy and looks about the same from either side.
Place the bonnets together, right sides out.
Using white, place a slip knot around the hook -- in preparation for the first sc. Make the first sc at the bottom of the back of the neck flap. Continue to sc the bonnets together. Use white to sc the edges of the neck flap together and blue to sc the edges of the rest of the bonnet together. On the sides of the bonnet, you'll be making 3 - 4 sc's per 2 rows, approximately 1 stitch in from the side. When changing yarn, do the last yo of the sc with the next color (as in the pic) -- since the loop produced at the conclusion of the stitch will be the top of the next stitch.
On the top of the bonnet, you'll be making 1 sc per st, placing the hook through the top loops (all 4 of them) for each stitch.
Stop when reaching the bottom of the front of the neck flap. Do not break yarn. However, weave in all the other ends. Next week will be the conclusion -- scalloping.
By the way, this pattern is turning out so well that I've added it to Ravelry under Sunbonnet Potholder.