The specs on the physical dimensions of a hard drive and DVD drive are deceptive. Hard Drives are actually 4" wide, but the spec calls it a 3.5" form factor. The DVD drives are actually 5.71" wide, but the spec calls it a 5.25" form factor. Go figure. It really doesn't matter since the computer case gets it right.
What it does say is that hard drives are not as wide as DVD drives, which means there has to be two cage sections in the case one with a small width for the hard drives, and a larger width cage for your DVD drives.
The problem with installing drives is that there are several different methods of installing drives in the computer case depending on the manufacturer and the size of the computer case.
Some basics, if you look at your hard drive, you'll see 3 screw holes along each side. Most of the mounting methods consists of screwing screws in these holes through holes in the case, which holds the drive. Some cases have pull out cages that make drive installation relatively easy. You install the drive in the pull out cage and then slide the cage into a slot in the computer case. I recommend you check out the installation booklet that comes with the case to see the recommended method of installing hard drives in that particular case. Each case is different. The screws you got with the computer case are the screws you will use to install the hard drive.
DVD drives install slightly differently. The front of the DVD drive has to go through the front of the computer case so you can load the DVD. To do this install, some cases require you remove the front panel of the case, which is usually held on with plastic latches. You'll have to remove a plate in the front of the case for the DVD drive to come out the front. You don't have to do that with hard drives, since they are entirely internal. Because of this, there are different methods of installing DVD drives. Some DVD drives require you screw in slider rails that you can move the DVD drive back and forth to align it with the front of the case. Some slide in from the front and are locked in place by a sliding brake mechanism. Others have the same screws in the side as with the hard drives. Again check your installation guide that comes with the case for you case particulars.
Let's talk about the connectors on the back of the drives. If you purchased SATA drives, your all set. Just install the drives in the case with the interface connectors on the back of the drives facing the motherboard so we can hook up cables.
If you did not, that means your hard drives and DVD drives use an IDE interface. If you have IDE drives, you have to be aware of the jumper on the back. This is a 6-pin connector on the back consisting of two rows of three pins. There is a jumper over two of the pins. If you have more that one DVD drive one of the jumper should be connecting the "Master" pins on the connector panel, and on the other DVD drive, you'll have to move the jumper to the "Slave" pins. If you have only one drive, the pins should cover the "Master" pins. The pins are usually labled, if not check your drive installation manual. The same goes for the hard drive. The good news is with SATA drives, you don't have to worry about that anymore and you shouldn't see any jumpers at all.
Install your drives, will do the cable hook ups in another post.