General Hints for
1. Keep it simple. All the computer is doing is looking for a string of letters and spaces. The longer the search string, the less likely it is to find an exact match. If you can keep it to one word, such as a surname or the root of a word ("child" will pick up both "child" and "children"), so much the better. If you only want "child" consider using "child" with a space after it, realizing that that will eliminate "child" followed by either a comma or a period.
2. If you want the words to fall together exactly as written, place quotation marks around them, such as "9th Texas" or "Ninth Texas."
3. Remember that during the 19th century, American spelling had not quite standardized. British spellings ("criticise," "woollen," "traveller") were still in fairly common use. In order to find everything, try each spelling separately. Names and words may also be misspelled in the original, not to mention transcribed incorrectly. Try common misspellings, such as "dies" for "dyes."
4. Be aware that the same word can be used to mean several different things, and can even be a surname. Letter combinations creating small words may accidentally appear in the middle of longer words.
5. There is no true subject approach in these files. If the word was used in the original, it appears here. If it didn't, it won't. Therefore, think of all of the synonyms and closely related words you can for your concept, and try each individually. For example, use both "recipe" and "receipt."
6. If you have a broad concept, think also about the individual components. For example, if you are looking for what soldiers wore on their feet, think shoes, boots, socks, and stockings, and even try barefoot or bare feet. If you are looking for dress, think also about robe and gown, or even costume. If you are looking for information on the 22nd Texas Infantry, you might also want to look for "Hubbard" (the colonel) or "Walker's" as in Walker's Texas Division, or "Mansfield" as in the battle.
7. For a really broad concept, such as "refugee" there may be no substitute for just scanning down the individual files and copying and pasting into a blank document. Refugees may not always be called refugees--you may be looking at reports of abandoned towns or citizens being banished from cities. The people involved may be considered refugees nonetheless.
8. If in doubt, scan through a few pages of a selected file to get a feel for how nineteenth century editors organized information and wrote articles. You can then try again once you get closer to their original mindset.