It would be great if we all had a detailed guide to show us the upgrade steps specific for us. A step-by-step guide is a little more complicated than you might think. Without knowing your environment, any plan anyone will give you will be at least a little generic.
However, Microsoft has done a pretty good job at providing the steps. You will need to make your decision on how you will upgrade your environment. Your choices are gradual upgrade, in-place upgrade, and database migration. Basic concept: if you have a highly-customized environment, you will have a more complicated, but not impossible, upgrade path.
Instead of rewriting what has already been said, I suggest following Joel Oleson’s advice in his post labeled Best of… Upgrade and Deployment Guides for WSS v3 and Microsoft Office SharePoint Server 2007.
If you follow the steps listed on Microsoft’s TechNet site, you will probably have a successful upgrade. But, you have to read it all!
I suggest start at the beginning: Upgrading to Office SharePoint Server 2007
Here are the links to the sections below the link above.
If you want to do this correctly, you will need to read the content and make an informed decision on your path to upgrade. I have done upgrades and it can be a pain. But once it is done, you will love your new environment!
Response to a SharePointU post here: http://www.sharepointu.com/forums/p/2800/8019.aspx#8019