There is many ways to join granny squares. The simplest one is joining with single crochet stitch, when you keep squares right side inside to each other. The joining will be convex on the left side of the needlework. To enhance the edges in the corners of the squares we make 2 single crochet instead of one.
Right side. We are joining squares the same color as the last lap to make seams invisible.
I wanted to enlarge the blanket, so I chose a different method of joining. We do a loop, such as to start crocheting. We arrange squares right side to each other, if we want to have a braid on the left side and the left side to each other, if you want the braid visible on the right. It depends on your taste. Crochet hook with loop put in corner’s square and make 3 chain, yarnover twice, put in in the second square, drag another yarnover and make triple crochet. * Put in triple crochet in the first square, but left the last loop on crochet hook, so we will have 2 loops on crochet hook. Put in incomplete triple crochet in the second square. We have 3 loops on the crochet hook, drag the thread on all. * We repeat it to the end.
Two videos below show other ways, it can help you understand the hardest way.
For the last two laps in the square and joining I used to more fluffy yarn.
Enjoy your crochet!