The classic way to proof is to open the text in your editor or word processor, and just start reading carefully.
This method has received a major boost since editors and word processors have added a feature of showing squiggly red underlines under words not in their dictionary. While this is very useful, you still need to read carefully, since not all errors produce misspelled words. The classic, and very common, example of this is scanning "he" for "be". These visual spellchecks also commonly do not check words beginning with capitals. Capitalized words are commonly names not in the dictionary, and when checking of capitalized words is switched off, they will not query "Tbe". Other errors that a spellchecker doesn't look for include missing spaces, mismatched quotes and misplaced punctuation. For these, you can try gutcheck [P.1]. And of course, no automatic check will find omitted lines or words. Worse, spellcheckers will query words not in their dictionary that might be quite correct, and this can be quite troublesome when dealing with older texts or dialect.
Still, if your concentration is up to the job, scrolling through a text with non-dictionary words underlined in red is a fast and effective way of giving a text the final once-over.
Volunteers have also used other techniques for proofing. Some people can't sit at their screen and read for hours; many people don't want to.
Some people just use the good old-fashioned method of printing out the text to be proofed, and blue-pencilling the mistakes.
It is becoming fairly common now for people to load the text onto their PDA, and read it from that. Mistakes found can be bookmarked or jotted down and fixed when they go back to their PC.
Getting your computer to read the text aloud is a very effective way of achieving high accuracy. Modern PCs have audio capabilities built in, and it is possible to find free or cheap shareware "read-aloud" text-to-speech packages for just about everything. Some PDAs are also capable of doing text-to-speech.
The first time you try text-to-speech, it will probably sound and feel a little strange, but you will quickly learn to hear errors in words. This can be very effective, but you should have given the text at least a light proofing before you begin; it is hard to deal with a high number of errors using a text-to-speech method.
When proofing by a speech program, you either set your text-to-speech program to pronounce all punctuation, or, if that is not possible, you make a special version of your text to feed it, first doing a global replace of "," with " comma ", ";" with " semi-colon ", and so on. Mark a block of 500 to 1,000 lines for reading aloud, and set the reading speed to whatever is comfortable for you. Then you sit down with the original book in front of you, and listen. When you hear an error, mark the place in the text with a light pencil. Stopping the reading at every error, editing the text and restarting is possible, but it breaks the flow, and ends up taking longer. When the reading is done, go to your keyboard and correct the errors found.