Okay, so now you know how to add squares along the outside edge, now lets go over how to add a square to the 'inside' of the project.
As you can see that the lighter blue squares will support the cast on stitches for the new darker blue square! Begin by picking up the stitches along the edge. Always beginning with a stitch in the last stitch of a completed square.
Pick-up evenly along the garter stitch edge picking up one stitch in each bump. (You may have to fudge the corner) Sometimes there is a bit of a hole but it can be tidied up when sewing in the ends.
Place a marker at the corner where the 3 completed squares meet. Then pick up the same number along the other square.
Knit one row and then complete the square.
The nice thing about this way of attaching as you go means you can add squares fairly randomly and create a blanket of left over yarn as it becomes available.
I hope this makes some sense, this tutorial has turned into a project!
See you soon.