There are several ways to accomplish this. When making a listing, you can assign a flat-rate shipping fee to each of the countries or geographical areas that you are able to ship to. Alternatively, you can assign a fixed flat rate shipping cost to all international orders, regardless of location. When negotiating on the price of an item via the offer system, you will also be able to adjust the shipping rate based on the location of the buyer.
You have the ability to change your shipping policies in your account settings at any time, however, changing these policies will not affect orders or listings that have already been posted -- these will need to be edited individually.
The most important thing to remember when listing international shipping as an option is to be as specific as possible. We encourage you to do some research about the cost to ship to certain countries, and can calculate an appropriate flat-rate for different areas.