I followed this guide to replace the HDD on my Mac Mini (2009, aka model A1283):
The most tricky part is probably opening up the Mac. As I did not have a putty knife at hand, I used a rack cage nut mounting tool to create an opening, and a very thin flathead screw driver to pry the case off, as explained in the guide.
EDIT: added picture of tool.
Send a link to this post to yourself or a friend.