Now that you know the preferred minimum size for the external hard drive, you're ready to set up Time Machine. Start by making sure the external drive is available for your Mac. This may mean plugging in a local external or setting up an NAS or Time Capsule. Be sure to follow any instructions provided by the manufacturer
Most external hard drives come formatted for use with Windows. If that's the case with yours, you'll need to format it using Apple's Disk Utility. You can find instructions in the 'Format Your Hard Drive Using Disk Utility' article.
Configure Time Machine
Once your external drive is formatted correctly, you can configure Time Machine to use the drive by following the instructions in the 'Time Machine: Backing Up Your Data Has Never Been So Easy' article.
Using Time Machine
Once configured, Time Machine will pretty much take care of itself. When your external drive gets filled up with backups, Time Machine will start overwriting the oldest backups to ensure there is space for the current data.
With the 'twice the Users data' minimum size we suggested, Time Machine should be able to keep:
Hourly backups for the last 24 hours.
Daily backups for the past month.
Weekly backups for the past 1 to 6 months, depending on the size of the drive.