Dating the work and sending a copy to yourself or putting it in your lock box does not protect the design. It is obvious from all these posts that you have an original work. Sure, the law says once you do the work it is copyright protected. However, if you should have to go to court and prove it or sue because someone else did a pattern, you might win but you will have to pay the court costs which could be thousands of dollars. I know this from other cases dealing with quilt copyrights. It is easy and inexpensive to get a registration. I would not wait until you have all the instructions written. Register the design now and then again when the instructions are finished. Just think, someone may have already made a copy of your design and registered the copyright claiming it as his/her own.