Using 1 C of kombucha tea either from your own batch, from a friend or you can even use store bought in the refrigerator section, allow it to come to room temperature if it was refrigerated.
Pour it into a glass jar(s) of your choosing. Whatever the size of the width of the jar, is the size your scoby will become.
Cover with a thin cheese cloth or paper towel and secure it with a rubber band. Your kombucha needs to breathe for the yeast to form the scoby. If it can't breathe, it will die and no mother scoby will form.
Allow this to sit at room temperature undisturbed for a minimum of 14 days. It will form layers. The longer it sits, the bigger it grows.
Check out my other links of sharing your scoby and how to brew more.