Firstly, make sure that you have a clear idea about what to draw. Go through the book that requires pictures on it if it is not written by you.
Next, identify the nouns and symbols in the text. These will form the illustrations that the children would love to see.
After this, make a couple of rough sketches and have a discussion with the writer about it. If you feel that the sketch you have drawn needs to be improved more then make another sketch. Once done finalize all the sketches.
Once completed, make a final draft using high quality material. Add suitable colors in it.
Finally scan the draft and save the scanned file on your desktop. Make sure that you have saved it in various image formats.