1) Backup & Copy site files & database to a Local Site and/or Staging Server, including .htaccess, robots.txt, sites directory (settings.php), contrib modules, custom theme, & files directory. Setup a "Legacy" version of the site to reference original config settings and content without disturbing the live site. Setup a "Working" version in which to perform upgrade.

2) Perform a Module Audit and create a spreadsheet of the audit for the for the site. This will become a useful reference document for the site to help note which modules may have been orphaned, replaced, etc.

3) Switch custom theme to a core theme before upgrading to ensure there are no conflicts during upgrade. For best results use the "Garland" theme.

4) Disable all custom and contrib modules and then remove all Drupal 5 site files from the Drupal root directory.

5) Upload new Drupal 6 files and directories into Drupal root directory. Copy the backed-up files and sites directories into the Drupal root directory and recreate the modifications of .htaccess & robots.txt in the file versions using the back-ups from step #1. Modify the new settings.php file to point to database. Run update.php script to update core database to the new Drupal 6 install.

6) Reinstall all avail contrib modules. Ensure all custom modules match Drupal 6 formatting. For a major update such as from D5 to D6, modules from previous versions will not be compatible and updated versions will be required.

7) Enable all core modules per settings on "Legacy" site and re-run update.php. Begin re-enabling contrib modules a few at a time and re-run update.php. Run only a single module update at a time in case errors are encountered along the way,this way they are easier to pinpoint. For example: Ubercart has 18 sub-modules within it and so this module will need to be enabled and QA tested independently from the others in case errors are encountered. If errors are encountered, then will know what they are in relation to, rather than having to pinpoint the problem by searching all of the site's modules later.

8) Upgrade/update custom theme. Create a .info file for the theme and make sure all regions and style sheets are declared within.

9) Rebuild and/or reassign all Views and Content Blocks to proper Regions.

10) QA test theme update, content, all views, pages, menus, blocks, etc.