Step 1: Gather your materials: finished granny squares, a blunt needle, and your seaming yarn (I used a contrasting yarn for demonstration, but you may want to use matching yarn). Lay your granny squares side to side with the front side facing up.
Step 2: Insert your needle into your first crochet stitch from the back to the front.
Step 3: Repeat Step 2 on the opposite side. Continue this process on each stitch, alternating between sides. It’s sort of like lacing shoes.
Step 4: Ready for that seam to disappear? Lightly pull on both ends of your seaming yarn.
Step 5: Repeat with all squares, and that’s all there is to invisible seaming! Remember to weave in your ends and enjoy your new granny square project.
Source: http://blog.lionbrand.com/2012/04/19/how-to-invisibly-seam-granny-squares/