One of the easiest ways that I have found is to regress to my childhood and play with aSpirograph.
For those of you who have never heard of the Spirograph. It is a toy from the 1960's which is a system of cogs and wheels which allows you to draw complex patterns easily.
|A basic Spirograph design|
After creating a design on paper it is simple to scan your creation into your computer using a flatbed scanner or even a 3 in 1 printer.
You can add extra color by using a painting program such as paint or just upload the design to Zazzle and begin placing it on the products that you feel will work with your design.
For instance I used this design on a tie.