1. Open the image you want to use as pattern.
2. Then, go to Edit > Define Pattern. Type in the name you want for the new pattern and click OK.
3. Now, we want to use the new pattern as a background in our new project. Open a new document, whatever the size is up to you.
4. Go to Layer > New Fill Layer > Pattern. You'll see this small window.
Click OK and you'll see another window for you to select the pattern you want to use. Choose the pattern you created just now and click OK.
And you'll have a nice background for your new project.