1. Open the image file in Photoshop.
2. Go to Edit > Define Pattern. A small window like below will appear. Type a name for your pattern and click OK.
3. Open a blank canvas. It can be of any size. You're gonna fill this canvas with our pattern.
4. Working on the blank canvas, go to Layer > New Fill Layer > Pattern. You'll see below pop-up window. You can type a name for the Fill Layer or leave it at its default setting and click OK.
5. Then, below window will pop up. Select the pattern you just defined in step 2. You can adjust the scale of the pattern if you want. And click OK.
6. Your canvas is now filled with the pattern. Below is some examples of a same pattern set at different scale.
Scale at 50%
Scale at 100%