Vector is best. But you can also do a bitmap. Make sure that the artwork is at least 300DPI . Bitmap Size: 11,7 inch x 16,5 inch. Don't use gradients other than shown in the example above. You have to use rough halfones to simulate a gradient.
Please use 5 spot colors only. You can also add white as sixth color - highlight white.
Don't let colors overlap each other and put each color on a single layer for screen pritning purpose.