0. Do you really need to create a WebQuest from scratch?
Read http://webquest.sdsu.edu/adapting/index.html to learn about adapting existing WebQuests.

1. Read:
http://webquest.sdsu.edu/project-selection.html
Pick a topic that requires understanding, uses the web well, fits curriculum standards, and has been difficult to teach well.

2. Study:
http://webquest.sdsu.edu/designpatterns/all.htm
Select a design that will fit your topic. Download the student and teacher templates for the design you chose. Open them up in your favorite web editor (Dreamweaver, Composer, FrontPage, etc.).

3. Write up the Task in the student template and the Standards and Learners in the teacher template.

4. Read:
http://webquest.sdsu.edu/rubrics/weblessons.htm and
http://webquest.sdsu.edu/rubrics/rubrics.html

Complete the Evaluation section in the student template. Duplicate it in the teacher template and add any extra information needed by teachers.

5. Read:
http://webquest.sdsu.edu/searching/fournets.htm and
http://webquest.sdsu.edu/searching/specialized.html

Flesh out the Process section by finding a focused set of resources to provide the information needed by learners.

If you have any doubts about the legitimacy of a site, check it out with Fagan's URLInfo tool.
http://www.faganfinder.com/urlinfo/

Scaffold where needed with Process Guides.
http://webquest.sdsu.edu/processguides/

Check yourself:
http://webquest.sdsu.edu/processchecker.html

The process isn't always as linear as this, of course. As you work your way through the steps, you may need to go back and modify the work done in previous steps. By the time you get to the bottom, you're done! The most difficult part is choosing a design and task. The most time-consuming part is designing the process.