It's a skill that develops organically throughout the course of the challenge (closer to the end of it, than the start, so don't stress if it takes a second - it's an advanced thing). For now, all I'll recommend is being mindful of the angle between each line of your initial y-shape, and varying them, and doing the same with the lengths of each arm of the y. Paying attention to how those changes end up changing the overall box is a good start towards understanding rotation as a whole.