Here's how it's done in the Unity C# reference, for example:
        // Creates a rotation matrix. Note: Assumes unit quaternion
        public static Matrix4x4 Rotate(Quaternion q)
        {
            // Precalculate coordinate products
            float x = q.x * 2.0F;
            float y = q.y * 2.0F;
            float z = q.z * 2.0F;
            float xx = q.x * x;
            float yy = q.y * y;
            float zz = q.z * z;
            float xy = q.x * y;
            float xz = q.x * z;