Hi,
If you mean installing the drive from a hardware point-of-view, then in addition to the 2.5" HD you will need:
1. A 44-pin IDE cable (10cm long preferably)
2. A metal A1200 HD cradle
3. Two fixing screws
The cradle is used to securely mount the HD over the motherboard and metal shielding. Installation is really simple:
1. Screw the HD to the cradle.
2. Position the cradle inside the casing (it fits over the metal shielding towards the left-hand side of the case. There are four holes in the motherboard which the feet of the HD cradle stick through).
3. Connect the HD to the motherboard IDE header (positioned immediately to the left of the HD cradle)with the IDE cable.
4. When you put the keyboard back in the A1200 casing, you should notice that it rests nicely on the three upwards-protruding arms of the hard disk cradle, if it has been positioned correctly.
I did the above just the other day. If there is an easier explanation, then someone else might like to correct me!
Steve.