You may have to collect promised donations after an event has taken place, or you may come across people who want to donate after the event. Your personal page remains active for a number of months after an event, so you can accept these donations.
The process is no different than normal. The donor goes to your personal page, selects the donation method and fills out the required information. If a donation is in the form of cash or a check, you must get the actual cash or check before that donation counts towards your goal.