Bell Curve is typically used by companies to track and manage performance rewards so that the overall rewards are distributed based on the performance of an individual. For a bell curve to work, companies need to devise a proper rating system where in the employees are rated based on their performance. Once the ratings are plotted, the bell curve helps identify how many (percentage) employees have been rated in various scale. The bell curve concept states that the majority of the employees should fall in the average zone with a few outstanding and poor rating.
Outstanding, average and poor rated employees are rewarded differently. Companies ensure that the number of people falling in the average zone is equal or greater than the total number plotted on the outstanding and poor scale.
Another interesting way to evaluate the employees is a nine box grid.
There are many HR systems (I can remember mentis hrms as I have used it before) that can help you plot bell curve automatically. If you use a software like Mentis this exercise becomes easier otherwise you can use excel.