1. Open Photoshop. You need to open an image from which you will be a creating pattern.
2. Filter > Pattern Maker
3. This will open the Pattern Maker dialogue box with the Rectangular Marqee Tool selected by default.
4. Drag Rectangular Marqee Tool on the portion of the image from which you want to create the pattern. (marked with white circle)
5. Click Generate.
6. This will create the pattern which has to be saved.
7. At the right bottom of the dialogue box you will find "Tile History".
8. Select "Saves Preset Pattern" (marked with green circle)
9. Click Cancel to close the window.
10. We will use the pattern now.
File > New
Go for any size of the file with RGB mode on.
11. Edit > Fill > Use (Pattern)
Open newly created pattern from the custom pattern flyout menu. Click OK.
This will fill canvas with the pattern.