How do you deal with firing an employee and what were the most common reasons?

The world of business is not just about those winning moments of raising money, hitting a 6-figure MMR or winning prestigious awards.

Sometimes, for various reasons, you may need to inform someone that they are no longer working for your company.

How do you assertively give someone notice?

Can you share your experiences, how you proceeded and what were the most common reasons for being fired?

(Note: I have somewhere read that some companies hire specialists to do this job for them when they do not have an HR department.)