Note: I recommend you to read the "Define Pattern" tutorial before this because in this tutorial I won't explain the way of defining patters.
1. Create a new project
Its dimensions are totally up to you.
2. Select an area
In this tutorial I will use the "rectangular marquee tool", but you can use the "Pen Tool" or any type of "Marquee tools" and so on...
3. Filling the area with the pattern
Right click on the selected area > Fill. Then use these settings what you see on the following picture.
Final result
