Start with a canvas size of 3000px x 3000px at 300 dpi.
Import your motifs or artwork; I typically use watercolor PNGs with transparent backgrounds. Place them on the artboard in an fashion you like.
Increase the size of the canvas to 9000px x 9000px.
Group your artwork into a folder.
Duplicate it and rename it however you wish. Hide the original artwork folder and don’t mess with it.
Convert the duplicated folder into a smart object.
Now duplicate the smart object and go to Filter->Offset. Add and subtract 3000 so that you fill the artboard with 8 copies of the smart object.
Double click the smart object. It will open up a new tab. Go to Image Size, and be sure that the file is 3000 x 3000 px. If it isn’t, resize it.
Now you can play with the placement of all of your artwork. When you’re ready, save the file and go back to the large canvas and see how your pattern has transformed!