Save xml file by path from user select

I'm success to save objects in xml file by path that I given. now I want to save in path that user select( I want FileDialog Box that the user select in the computer exactly where to save and can to creat/replace new file.xml)

I try to do by this guide but I don't understand clearly so it doesn't works. http://ift.tt/14KVagl

I will be more that happy to get some help. Thanks and have a good day. :)