There are various knotting and tying techniques used for crochet braids, and some are better (more secure) than others:
Loop once then tie off once with one or 2 knots. This is the most insecure means of joining your synthetic hair onto your cornrowed hair and the most likely to work its way undone with time.
Pull one strand through the front of the loop and pull one strand through the back of the loop. Twist loop and pull through both strands and pull tight. Another knot can be tied for added security.
Pull both strands through loop. Take left strand under right stand and right strand through front of loop. Pull tight. You can also tie another knot after this for added security.
Tie knots of your choice. Then tie a strand from each adjoining section of hair together. Some have reported good success with fastening the synthetic hair to their own hair like this.
Pull strands through loop one time, and then twist and loop at least 2 more times, then pull tight.