Instructions
Read up on the subject. Antique flags are valuable, however, there are replicas and forgeries in the marketplace. Reading guides like "Flags of the World: Past and Present: Their Story and Associations" for the collection of world flags, "Chapman Quick Reference Guide to Nautical Flags" for nautical flags and "The Flags of the Confederacy: An Illustrated History for Confederacy Flags" would be helpful.
Go to Country Living's website and read their article on collecting American flags. It is a quick reference to the various flags: Civil War flags, campaign flags that politicians used as they were running for office and World War Two flags.
Visit Jeff Bridgman's site. It has a wealth of information about collecting flags, including what to look for when collecting, how to preserve your flag and how to start your collection.
Read David B. Martucci's Antique U.S. Flag Assessment Guidelines. A vexiollogist (someone who studies flags) since 1966, Martucci provides a valuable resource for the flag collector. He also provides an assessment service for antique flags.
Find your flags at live auctions or antique shops. There, you will have the opportunity to study the flag carefully, looking for such clues that can date it, such as the manner in which it was sewn (machine or by hand), the fabric and its true condition. Buying flags on auction sites would be a gamble as you would not be able to be as thorough in your inspection, and the risk of buying a forgery is much higher.
Have your flag purchases assessed by an expert to ensure you buy the real thing and not a forgery.
Preserve your flag properly so that it can be enjoyed by future generations. Store it in an airtight display case if possible.