Guess: "Hypothesis" is just a fancy word for "guess." To solve a cipher, start with a simple hypothesis, and choose some letters to test it out. For example, you could start with the hypothesis that the letters XYZ stand for THE, then you would set each of the letters X, Y, and Z to be T, H, and E, and then you would look at the result. You can do this quickly, and if it turns out that your hypothesis is not confirmed, you can easily undo it and try something else.
Start with THE: First, guess common short words. Start with one word. In this example, we have started with the word THE. Now look at the rest of the cipher, and notice the places where the letters T, H, and E have now been filled in. Do those places make sense? If not, guess again.
Word combinations: Also notice word combinations, and think about what would not make sense. If you see a three-letter word, followed by a one-letter word, how likely is it that the three letter word is THE? If a guess leads to a combination like THE A, then guess again.
Single-letter words: The one-letter words I and A are good clues. In personal messages or quotations, I might be a good guess, but in most other writing, A is probably a better guess.
Common letter positions: Keep in mind that some letters like E often occur at the ends of words. (But, of course, not only at the ends of words.)
Re-read often: Every time you make a guess, do a quick scan of the entire cipher, looking at what possible words (or possible nonsense, telling you your guess is wrong) have been revealed.
Giveaway word patterns: Make use of not just short words, but also other hint words: number word combinations (3 YEARS OLD, 3 YEARS AGO, 3-YEAR-OLD, JAN 25, 2009), apostrophe words (CAN'T, DON'T), double-letter words, and hyphenated words.
Possessive S words: When guessing an apostrophe word, don't forget about 'S (possessive).
Check word lists for ideas: Use the lists below as a reminder, especially paying attention to the most common words, those at the beginning of each list.
Using the lists: Read the "Expert Tips for Using the Lists" below for more ideas.
Don't use the longer lists!: Most ciphers will be solved more easily without using the longer lists (like some of the lists of words containing doubled letters). That's because the list of possible words is often just too long. You need to make progress solving the cipher by other means first, before you'll have some letters to help you narrow down the choices in a long list. And, by the time you've done that, you'll be able to guess more letters, which lead to more words, and more letters -- making the lists not as useful as they seem. But do use the letter list, the short words lists, and the apostrophe words lists; those are very helpful. The other lists are just there in case you want them.
Not every word is listed: The lists only contain common words for of certain types (short words, apostrophe words) and then most words for another special type: words with doubled letters. Many words do not fit into these two categories, and are not included in the word lists, because there would be way too many possibilities for any such list to be useful.
Frequency Order: The lists below show the most common words at the beginning of each list. This is very important! If you have a choice, guess the more likely word, meaning the one closer to the front of a list.
Patterns: Use word patterns to narrow down choices. Look for not only double letters that appear together, but also places where the same letter is repeated in different positions in the word.
Known Letters: When you are sure you know a letter, use that letter when you examine a list, thereby narrowing down your word guesses.
Start Small: Don't start looking for long word matches too early in your cracking process. Wait until you have a matching letter or two, unless the list of matching words is especially short.
Common Words, Common Sense: Use common sense to choose the most likely words (remember, those at the front of each list). Both LOOK and LEEK have four letters, a doubled letter in the middle, and start with L, but only one of them is very common, so it's a better guess.
Topic: If you have an idea what the text might be talking about, you can start eliminating words that don't fit that topic, and looking for words that do. For example, if the topic is astronomy, TELESCOPE makes more sense than TELEPHONE.