If you are illustrating a picture book, or a graphic novel, rough sketches and dummy books are the way to go when in the beginning phases. You need to ensure your illustrations work well as a unit with the text, suit the page orientation and have proper pacing prior to creating the final art.
There is the upcoming Smart Dummies - A Picture Book Challenge that can help guide you through this process. It starts September 1, 2015 and runs for the whole month.
For further dummy book info, you can also refer to:
Specs, bleed and fold
Once you have your dummy book, and are ready to start the actual illustrations, the next steps include the following:
o Size is always determined by the art director.
o Double the size is twice or ‘two up’. It is two times the diagonal of the finished size.
o Remember that tiny details are lost when an image is reduced in size.
o Ensure digital images have a resolution of 300 to 600 DPI (dots per inch) .
o Ensure digital files are in the required mode (CMYK, RGB).
o Refer to Why size matters when it comes to digitizing art.
o Never put small details such as hands or faces in the center.
o Keep text at least 2.54 cm (1 inch) away from the fold.
Final digital steps
Whether you work digitally from the beginning, or scan in your final image, ensure the following:
Here are some templates that are available online: