The manufacturer's website or the manual for your hard drives should have fairly detailed instructions.
For example, the Seagate website:
http://www.seagate.com/support/kb/disc/howto/ata_install_tshoot.html
They're probably better at explaining things than me. (:
Do the drives look similar to the one in the photos in
>>4 ? If they don't, then quite a bit of what I say next may be irrelevent.
When adding or removing hardware from your computer, make sure the power lead is removed, and don't touch any circuit boards. Touch the case of your computer to earth yourself before handling components.
Firstly, turn off the computer and remove both of the hard drives. You should see a block of 8 pins, as shown in my previous photo. There may be one or two pieces of plastic covering some of these pins (known as a Jumper). You can remove a jumper, and put it over a different pair of pins. Set one as Master and one as Slave, as directed by your manual or the label on the drive. You can also set both to 'cable select', which automatically sets one to master and one to slave depending one which way round on the IDE cable you put them. I'm not sure which way round it is, though.
Once you've done that, you can put them back into the computer. The cable which goes from the motherboard to the drives should have a connector on each end, and one in the middle. Sometimes they just have a connector on each end, in which case you'll need to buy a new cable from a computer or electronics store. Anyway, connect both drives to the same cable, and reconnect the leads from the power supply.
When you next turn on the computer, you should see the drive listed as 'primary slave'. If it doesn't seem to be found (either during the boot process or you don't see it in My Computer), try rebooting and going into the BIOS setup by pressing F1 or Del as soon as it starts booting (it should say something like 'press Del to enter setup). If you have the documentation for your motherboard, read that and it should tell you how to add hard drives. Usually, it's under something like 'Standard CMOS settings' or something. Set Primary Slave to 'Auto' if it's set to None, then save and exit. It should then be detected.
I think that's everything. Maybe someone better at explaining things will add something later.