In most of projects there will be a moment, when we need to join a new yarn. There is plenty ways to make it. The main idea is to make a knot that can’t be untied.
My favorite way is the one shown in the graphic below:
I found it on the http://manualni.pl/ The image clearly shows how to lead a thread. Always tighter the knot and see if it works. If not, we try again, until we succeed.
I love that idea: https://www.youtube.com/watch?v=-nq_7EXTWHE
Another way, but without a knot: http://blog.loveknitting.com/join-yarn-without-knot-leaving-ends/
You can also use a needle. The description is in polish, but pictures are clear: http://manualni.pl/jak-polaczyc-konce-wloczki-bez-wezla/
I hope so you will find your favorite way.