Open your favorite image creation or drawing program on your computer. If you are limited to a browser, you might try the Online Image Editor or similar (though I have not used it in my example). For Windows, I am a fan of Paint.Net.

I grabbed a ruler to decide how wide and long I wanted my design. I chose 0.5x1.0 inches roughly which I refined to 12.7 mm x 29.21 mm through the design process.

You don't want to make things too big if not necessary to save money. 

Set the image dots per inch or equivalent fairly high to have a good resolution printed on the 1200dpi printer.

Draw Your Design

Draw your design in the image editor. I suggest starting with a outline of how big the item will be and what shape it will take.

I made a rounded rectangle in black and made the corners not in the outline transparent. I then made black lines in the rectangle in the style of Mondrian. Finally I filled some of the rectangles with color. Rather easy for my design.

You may wish to import a bitmap from somewhere else: a school or team logo, or a pet's picture. It's your choice (just be careful of image licensing issues). In my case, Mondrian's paintings are older than 75 years and mine are "Mondrian inspired," so they are doubly ok.

When you think the design is done, save it in PNG (bitmap) or SVG (vector) format, the formats that EasyEDA Pro supports for color PCBs.

Getting Fancy: Having Copper Show Through

My original goal was to have the yellow not be printed but have that be the golden color of ENIG, the gold plated copper that is the only finish JLCPCB allows for exposed copper. To prepare your bitmap for this, use the erase function in your image editor to make the copper part transparent also.

It doesn't come through, but the upper left and lower right boxes have been changed from yellow to transparent in the PNG file.