Make a list of everything that will be covered in the field guide. The basis for your field guide can be broad or narrow. For example, students in Seattle created a field guide expressly for Seward Park. However, it is easy to find that you have too much information for one field guide. If this is the case, set some criteria by which you can whittle down the amount of information. For example, if your field guide is about gems and minerals, you could narrow its focus to gems and minerals in the state of New Mexico.
Organize your information. Decide how many tiers of information you want in your field guide. Will it be divided into segments such as Northern New Mexico, Southern New Mexico and Along the Rio Grande? If so, divide your field guide into appropriate segments and then alphabetize all of the information you are presenting. Some field guides are strictly alphabetical, making the organizational part of writing the guide straightforward.
Decide what kind of facts your field guide will have. You should always include basic information about your subject, but will you include the scientific names of the plants, animals or minerals you are providing information about? How about fun facts? Think about your audience when deciding what information to include in your field guide. Books targeted toward educated people should include details that go beyond the basic information, such as the history behind a particular herb, or the chemical composition of a mineral.
Obtain photographs of the items you'll be providing information about. In the case of a field guide, a photograph is worth far more than the proverbial thousand words. You can buy photographs from stock photo companies that illustrate just about any subject.
Create an index for your field guide. If information about a specific subject is mentioned multiple places in the guide, cross reference it in the index. You should also create an index for the photos in the guide.