Just go to the post office, properly addressed, and come armed the value of your item. You can fill out the form there, and there's no extra fee to you. The buyer, depending on the value of the item you're sending, may have to pay some import taxes, but I find that international buyers are familiar with that, so it shouldn't cause you any problem.
There are ways you can do it without going to the post office, but if you expect just the occasional international sale, you might just want to go to the PO for now to keep it simple. You'll find it's really not difficult at all.