You can go to Tools i.e localhost:4502/miscadmin
Create a new Design--> Click on Design --> Either create a new folder or create a New page directly under design
Enter design name(generally your project name) and select design template and click on save.
Now go to Page on which you have create design dialog. Right click Page Properties > Advanced > Design and apply your design.
Its done now when you save content to design dialog it will be save in project specific path i.e /etc/designs/<page name> instead of /etc/designs/default.
Let me know if you face any issue. I will add this to my tutorial also http://www.aemcq5tutorials.com/tutorials/create-design-dialog-in-aem/