This article has shown a little bit of what it takes to convert a self built macro to a utility fit for distribution to others. Of course there are many more things to account for when one designs an add-in. Things like changing (and setting back!) Excel-application settings and designing a proper user interface haven't been addressed at al.
Nevertheless, the subjects discussed in this article will certainly get you started to create a professional looking add-in from a simple macro.
To those of you who need more in-depth information, I would very much like to recommend these books:
If you're more of a beginner, try the Dummies series:
Excel 2007 VBA Programming for Dummies. (there are separate editions for the older Excel versions)
Author: John Walkenbach
Revision author: Jan Karel Pieterse