He just said "double bow" before, but why did he say "thus a double bow" again? I wonder why "thus" was used here.
Answer from a Native speaker
He repeats "double bow" here to show that he has completed the "double bow tie". "Thus" is an adverb used here to mean as a result. After he has explained that you must tie a "double bow", he says "and thus, a double bow" to show that he has demonstrated how to tie a "double bow" and as a result, he has a completed "double bow". It is common for words to be repeated in this manner when someone is demonstrating something visually.