Begin by skillfully cutting thinly rolled puff pastry into squares, ensuring precision in your measurements. Proceed to generously spread a layer of jam on one half of each pastry square, leaving approximately a 4-inch margin from the edge. With utmost care, moisten the edges of the pastry using a brush dipped in water, then deftly fold over the other half of the pastry, firmly sealing the edges to prevent any escape of the delectable filling. Utilize a knife to trim the edges, bestowing them with an elegant scalloped appearance. For a delightful finishing touch, gently brush the top surface of each turnover with water and artfully sprinkle a layer of sugar upon it. As the stage is set, allow the turnovers to undergo a transformative journey in a preheated oven at 425°F, where they shall bake to perfection in a span of 15 to 20 minutes.