1. From each resource assigned to a task, get an accurate update of Actual work for that task.
2. From each resource assigned to a task, get an accurate estimate of how much work remains to be done for that task.
Then read the Project help topic (in P2003 Standard) called "Specify how task progress is shown in relation to the status date". (Note, however, that if you are trying to match task Actuals against a timesheet system then you cannot use this approach but must manually enter the actuals on a task-by-task, resource-by-resource day-by-day/week-by-week basis.)
3. Set the Status Date in your project (probably to today's date but you may have a different date in mind).
4. Update your project. For tasks which are complete you may find "Update Completed Tasks Quickly" useful.
5. You didn't ask, but I'd create a new baseline and use that in future project variance reports.