Every piece of crochet begins with a slip knot. It serves to secure the yarn to the crochet hook in order to make the foundation chain. A well-made slip knot should be stable and not stretch.
In order to make a slip knot, you need to form a teardrop-shaped loop from the yarn, crossing the ends in the place where they close, then interweave its free end through the center in such a way so that another loop is created. It is this loop which should be put on the crochet hook. The previous loop should be pushed downwards, thereby securing the yarn to the hook.