Follow basic instructions in Drupal.org Conversion Tutorial.
- Freeze all but text changes to site. (no configuration or code changes)
- Do (and redo until successful) a test conversion on test site. Some things may need changes once D6 is up (record for step 3 below).
- Schedule and do final conversion (site is down for this entire time)
- Modify to regain full function (site may be down for part of this)
- Full test and fix cycle