Most companies of any reasonable size should have a process for this. Check with HR. Things like turning in keys and such.
Have a HR person present to talk about separation process. Have final check cut and hand to the person. This also gives you a witness to any interactions.
Do not do it in your office. Reason: if the person goes ballistic or just does not want to leave you are stuck in your office with them.
Conference rooms are good.
Be direct and hones but respectful.
Have someone go with the person back to his/her desk to oversee/help the person get his/her personal stuff.
Usually IT will shut off access to the computer while you are in meeting with the person, but allowances should be made to get personal information off (for instance, I have photos on my computer that are used as my screensaver). I also have some scientific papers that are work and personal related. (I also regularly copy those to an external hard drive to make sure I have them in case I get canned or just as another back up).