Simply remove the thumbscrews holding each side panel in place to access the interior. The interior is quite modular, and there are a lot of cable-routing possibilities within the 750D. We also noticed the bumps to hold the motherboard in place. Having worked as a system builder in the past, this type of installation method does pose a higher risk of shorting the board out, so I suggest taping these up before placing your precious hardware on top. Corsair has done an excellent job in maximizing cable-routing and hiding possibilities, with a large number of hooks one may attach unwanted leads to during the assembly process.
You will also find four plastic frames that are used to hold individual 2.5" drives. Each of these can be removed without the need of tools because of a clipping mechanism. There is 30 mm worth of space behind the motherboard tray, which should be enough to route all excess cables nicely.
The bottom area in the front stores all hard drives. Corsair decided to place the two cages next to each other, but you may also stack these on top of one another. If there were a third cage, it would fit in here as well. While you could also have these hang off the 5.25" bays, which would keep the bottom area free, there is no way to properly secure such a constellation, only making it interesting to modders. The default configuration leaves both fans in the front nearly unobstructed, ready to push cool air into the interior to keep things as cool as possible. You will find the aforementioned 5.25" bays in the very top. Each has its own plastic tension lock for easy, tool-less installation.
The bottom PSU bay in the rear comes with a large air vent, which even gives long units access to as much fresh air as possible. Things could get a bit tight if you were to install a potent unit here while leaving the hard-drive cages in their default position. Above that are the nine motherboard expansion slots, each protected by an individual cover that is in turn held in place by a thumbscrew. The rear exhaust fan in the very top takes up most of the space.
Taking a look at the ceiling, it becomes apparent that a long radiator will block the very top 5.25" bay, which is a small price to pay for those who really want to make the most of the chassis.
All cables within the Obsidian 750D are sleeved black to go with the overall color scheme. Each individual plug is of the default variety, so you should be able to connect these properly without running into problems, making the most of the I/O.