In order to achieve control over our dynamics we need to work on three key elements with our bow arm. Bow speed, bow pressure and contact point (position between the bridge and fingerboard).
To play louder play closer to the bridge, draw the bow slower and apply more pressure. To play softer, play closer to the fingerboard and apply less pressure. Experiment with these 3 elements to find the ideal combination for what you are trying to achieve. Be your own teacher and ask yourself what makes you sound better and what makes you sound worse.