To enlarge the hole, put your finger into the hole you've already created and slowly start bringing it towards your body. You will need to use your other hand to brace the wall that is being created. Enlarge this to the size of the base you wish your pottery to be. This created wall would generally be about an inch to two inches thick. With the wall created, the cylinder should spin perfectly symmetrical. In all these steps always remove hands and fingers and take the pressure off slowly.