The first step is to determine the place for the party. If you want to save some money then choosing your home as the hosting area might be a good option. This will also give you greater control over the atmosphere and food. However, if you want someone else to worry about the arrangements and cleaning up later, then choose a restaurant.
If you want the party to be a surprise, you can arrange it at your workplace (with permission) and surprise your colleague when he/she comes in.
You cannot pick a date for the party suiting everyone, so the best you can do is to select a few dates and ask people to choose. The date selected by most people should be finalised. Usually weekends are the best for holding parties.
Since you will be inviting most people from work, it is better to use web-based invitation services. You can also co-ordinate over texts or e-mail.
Remember to mention the date, exact time and location in the invitation.
Although this is optional, but selecting a theme for the party can make it more fun. For instance, if your colleague is moving to Florida ask the attendees to wear tropical shirts.
Hosting the party at home means you will have to buy snacks along with beverages. Buy some balloons and hats for attendees to wear too.
Now if you are having the party in a restaurant, it is advisable to contact the restaurant and give them instructions regarding the menu and other arrangements. It is necessary to let the restaurant's management know who will be covering the cost of the bill.
Your work is done once the party starts, just sit back and relax. Let the party flow on its on. Half-way through the party, make a toast and ask other attendees to say a few words for the departing colleague.