Below are some tips that may help you in writing Text Equivalents:

Specific Font Styles

For images that contain text in some font style (WordArt or an imported image), use that text as the Text Equivalent. An example would be the Yahoo logo or the Google Doodle. The text equivalent for these items would be "Yahoo" and "Google".  The exception would be if you were not using the WordArt/Word Logo in it's native way. You would then use an appropriate text equivalent for that situation.

Navigational Symbols

For images that represent a navigational symbol (e.g. an upwards-pointing arrow), use the context the symbol is representing as the text equivalent. Examples can include directional as well as movemental descriptions. 

Character or Symbol

For images that are used to present a character or symbol within text (such as a Greek letter or small icon), write the word(s) that expresses the same thing verbally as the text equivalent. This is the same as with a Google Doodle. Write out what the words are saying. You may also include the language. The following, for the appropriate context, may use a text description that says, "Greek letters for Alpha Sigma Lambda."

Rich Content

For images with rich content of their own (such as a photograph or a graphic presentation) use text that performs the same function as the image, e.g. explaining verbally how the system works.

Avoid descriptions like "Graphic presentation of the ventilation system" as the text equivalent. This is where it is important to know what you are trying to convey with the image. Think about what the content would be if the image was not visible, and that will be close to your text equivalent. Tell someone the text equivalent you have come up with and see if they can visualize the picture/graphic you are working on.