First you need to mix 1 cup of water with 1/2 cup of all purpose flour. When it looks like milk it's ready for the next step.

cook at medium heat, stirring with a wooden spoon, it's easier to clean.

You have to keep stirring through the whole procces or it will get lumpy, and it's consistency won't be the one you need.
When you notice that it's starting to get thicker and changes color (like... it's cooked) set aside and let it cool.

Once it's cool, you can use it. It dries in about two to three hours, so you better use it fast. You can apply it with a brush or just spread it with your finger.