Flat Slip Stitch
This is the joining method I used on the Sunburst Granny Square Blanket (see the last picture below). It gives a flat yet clearly defined vertical and horizontal lines that frame up the squares.
Granny squares (blocked), hook and yarn for the joining.
I used 4 squares in different colors and a white yarn to join, so that you can see the stitches clearly.
Place the squares with right side showing up as we are going to work on the right side of the square. Make a slip knot on the hook to begin, insert the hook into the back-loop of the corner stitch of first square, bring a loop out and make a slip-stitch.
Note: If your squares has 2 chains at the corner (like mine), pick the chain stitch just before your first double crochet stitch on the edge that you are going to join.
Insert the hook into the same back-loop corner stitch from front to back.
Insert the hook into the back-loop of the corner stitch of the second square, from front to back too.
The white yarn should be always at the bottom of your work, now, hook the white yarn and bring up from the second square loop then first square loop.
Slip stitch to the first loop on the hook.
Repeat the process on the second pair of stitches and so on.
Continue until you reach the corner stitch right after the last double crochet stitch on this edge.
Bring in square 3 and 4, continue like the beginning of square 1 and 2 except that you don’t have to make slip knot onto the hook.
Continue joining stitch on square 3 and 4 till the end.
If you have more squares to join along the way, continue to do so until you reach the desired pieces of granny square in the row. fasten off.
Repeat the same to complete all the row seaming for the whole project before you proceed to join the column (vertical lines).
Follow the same instructions when you do the column join until you reach the end of your first square.
Make a chain stitch to cross over the row joint and repeat the same when ever you need to cross the following row joints.
Continue with the normal flat slip-stitch until you reach the end. Fasten off.
Here is the example of a 4-square join.
This is how the joint looks like at the back of the squares.