First go to the Advanced Search page. Sometimes you may miss in searching because of alternative spellings, so try searching separately using just one word in Author or Title. Read the Search Tips.
If that fails, you can Browse through the site catalog. Let's say you're looking for "The Wandering Jew" by Eugene Sue.
Go to the PG Find-an-eBook page: <http://www.gutenberg.net/find>
Once on this page, click on: "S" in "Browse by Author"
You should now see a list of all of the Authors whose last name starts with "S". Scroll down till you find the direct links to the Sue, Eugene works.
Click on the work you are interested to, then click on the file link found on the page you were brought to, Etext Card ID -3350- when selecting the work, as immediately above.
On this page, above the excerpt, there are download links:
Click on the link of your choice - plain text or zipped, and from ibiblio.org or other.
If you choose one of the mirrors, you are then brought to a new page, asking you to select an "Download site". Further details on how and why to choose an "FTP Site" are available on this page.
Select a site, and the file will be downloaded, or offered for download, depending on which format you selected and which browser you use.
If you can't find your text either way, the book has not been cataloged. If you know that the book has been posted recently, and maybe hasn't made it into the catalog yet, read the FAQ "How can I download a PG text without using the online catalog?" [R.3]
If even this doesn't help, don't despair! We don't have it, but it may be elsewhere on the Web. Go to the major search engines and try there. You can also try looking in the Book Search section of The On-Line Books Page <http://onlinebooks.library.upenn.edu/>, and if you have no luck with that, you might be able to find it listed as being In Progress somewhere on their Books In Progress and Requested page at <http://onlinebooks.library.upenn.edu/in-progress.html>.