This is a square for my black edged, multicolored granny square blanket, using a J/6.00 hook for 5 rounds and measuring about 5 in x 5 in a square.
You can find the pattern for this Basic Granny Square here. To change colors, I fastened off the first color then tied the next color onto the next corner space with a simple knot. To make the first chain of the new color, I just inserted my hook from front to back in the corner space where I just tied on the new color, yarn over and pull a loop up to act as your slip stitch. Start the pattern like normal from here.