Write a few ideas for writing-related illustrations as you put your book manuscript together. Use a notebook and write your sketch ideas there, marked with a number that corresponds with a number in your manuscript to indicate the location of the illustration. Consider illustrations such as feather pens and inkwells, typewriters or a writing desk with writing instruments scattered over it. The book is about writing, so keep the theme alive with your illustrations.
Sketch the illustrations in a sketch pad. Consider using nothing more than a pencil for the work. The book is about writing. The illustrations are a visual bonus. Color isn't necessary and not using color can save on the cost of printing. Pencil sketches also lend a more artistic quality to the illustrations, which will fit nicely with a book on writing. For writing examples, you may consider scanning pages of some of your manuscripts as illustrations as well, particularly to demonstrate the writing and rewriting process.
Scan your illustrations into your computer and run them through a graphic editing program before you insert them into your manuscript. This will allow you to size the graphics to fit the manuscript. It will also allow you to use some of the tools available in your editing software to clean up any flaws in your illustrations. Blending and smoothing tools will probably be used more than anything, just to prepare the illustrations for the book.