如何将数据写入xml文件

I want to store data (preferences) into an xml file , but I can't achieve it. I took a look at appledeveloper web but it seems not clear to me . I want to store the xml file on disk . My preferences that I created are some checkboxes (ih the user cheks the login box for example and I add this preference login to the xml file) , radio group, textfields can you give me a good tutorial or an example that shows that please?I want to store data (preferences) into an xml