I've made a video to explain a simple way of doing it, also covering making a straight border. The secret is to make the fancy bit of your pattern into a compound path first. Second step is to make some duplicates and to put them on the CORNERS first! It's much easier to do it this way, filling in the straight edges afterwards. Here is a free border file. NOTE - I made it very large but you can resize it however you like!
Going through loads of files when they don't have images attached is a pain in the wrist.
Opening and closing loads of files till you find what you want is a waste of crafting hours!
Now, I'm sorry for all you Windows users as this may not be possible for you, but Mac users can attach a screen shot to Studio files so that you can browse them more easily. Do this as a habit when creating files and it will make life a lot easier. It saves having to take a photo and resize it and you don't need to keep a copy in the file unless you want to. Your screen shot can be quite a small file too, saving your disk space.