Clamp the sides to the back with corner clamps to keep the edges flush and square.
To hide the screws, we're going to use timber plugs, which we'll make later. To accommodate these, pre-drill with a 9.5mm bit, then make a clearance hole with the 5mm bit. Apply glue for extra strength, removing any excess as you go. Make a pilot hole with the 3.5mm bit and then screw together with the 65mm timber screws.