The first step is to bring the images into Design Space. Navigate to the Upload button on the left side of the Canvas in your Design Panel.
Next, decide on your pattern. You can either bring in an existing pattern such as my mermaid or polka dot pattern freebies or you can create a pattern from the basic shapes panel.
Next, group the pattern line by using the group button at the top right and start to duplicate that again as well as offsetting your lines a little bit.