Shut down your machine, take power cords off, ground the machine (there are some fancy ways of doing that, but it should do if you touch the ground yourself and touch the metal cage of the computer) and don't wear clothes that could cause static electicity shocks. Shortly said, open your machine in a "safe" way so that you don't get lightning. Then install the harddisk to the cage, with all the screws that hold it in place, connect the cables and put back the cover of the machine.
Once the harddisk is physically connected, boot the machine. Fedora should pick it up all by itself. Then you can use (install trough package management, if needed) GParted to partition and format the disk - create one or more partitions on it if there are not already, and format it (ext3 is a good filesystem if you only intend to use it with Linux). After that it should be at your use.
The command-line way is to (as root - first command 'su -' to become root) use cfdisk (or fdisk, if you feel so, but cfdisk might be easier) to make some partitions on the device:
Then you could use mkfs.ext3 program to make ext3 partitions (if you want them) onto the partitions you created. Something like
for the 1st partition of sdb, your secondary harddisk. sdb2 for the second partition, and so on. Read
for more options of that program.
Once it's partitioned and formatted, you can mount it with 'mount': first make a directory where you mount the harddisk's contents (where ever you like; here I show how to make it to /media/):
Then mount it:
mount /dev/sdb1 /media/my_new_harddisk
And there you go. Again sdb1 is for the first partition...
If you want to automate mounting it, add a new line to /etc/fstab - for example
/dev/sdb1 /media/my_new_partition auto defaults,auto 0 0
again read more about fstab to know the options.