Anyone with a dog knows that if you let them out to do their business you better be waiting by the door when they want to come back in. If you're not, it's a sure bet that they will be scratching at the door trying to get your attention and, the bigger the dog, the greater the damage. This instructable is a classic example of my "closing the barn door after the horse got out" but, hopefully, it will save your door before it meets the same fate as mine.
Step 1: Low Tech, But Effective
This is a very low tech project that is inexpensive and easy to make.
I built this dog gate from left over scrap lumber but all you really need are a few 2 x4's that you can rip on your table saw plus piece of 1/8" or 1/4" thick hardboard.
The size of the gate, both the width and height are variable depending on the size of your dog AND the size of your door. For that reason I am not recommending specific dimensions.
The next few steps will detail the basic design which you can follow or modify as needed.