Present the employee with the severance agreement, worked on by your HR manager, and walk through each section. Leave time for the employee to ask questions and make clear the time frame during which the employee has time to meet with their lawyer, as well as their last day.
Voluntary separation offers on the other hand, are not typically calculated based on years of service, but are rather a multiple of monthly salary (i.e., 5-6 months of salary) to ensure the offer is competitive and attractive regardless of tenure.