After all the glue from the last few steps has dried (24h), start to glue the front part. First unscrew the front part and mark the middle of a side.
Now apply some wood glue to the lower half of the front face and the frame and press it together with the help of something heavy, like water bottles or books. Make sure you don't load it to the middle of the front plate, because this will bent the sides up and buckle the front side. Alternatively turn the box around and load the back with something heavy or use supporting boards.