The clamps in the picture were being used to hold the upper back panel in place until I could properly connect it to the sides of the cabinet (see picture below for notes on this step).
The gap you see there will be for access to the computer and inner components of the arcade cabinet. I haven't decided if I'll hinge the front door of the cabinet or not. There are some advantages and disadvantages.
Everyone does things a little differently, but what I do is pre-drill the pieces first - I drill 2 or 3 holes in the side and these will allow me to place screws into the sides of the cabinet. I also drill 3 or 4 holes in the edges and these allow me to place screws into the angled panel and back angled panel (see next image for closeup).
Here you can see a closeup of the screws that will go into the upper back panel. I've already screwed the bracing piece into the sides of the cabinet.
Pre-drilling works for me because it's very difficult to drill those holes after gluing the piece in place. My electric screwdriver is small enough for me to get in there and tighten the screws down but the drill is too bulky.