I’m almost certain there’s a script that will ease the process of doing this, so I’d have a poke around in the scripting forum.
Also, I know from experience that it’s always wise to build in some overlap into each of the tiles.
If you’re going to do it manually, the following image I created a while back for somebody else with a similar question may help you understand the methodology.
Click on image for full-size version:
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http://www.pixentral.com/show.php?picture=1emnJwnrX6eXZrNuJy 3bQCRGztPEm0>