It's easy!
All you have to do is select a category, then think of words that relate to that category, with the first one starting with "A", then "B", and so on, all the way through to "Z".
If you can't think of one, don't waste time! Hit the "Pass!" button and move onto the next letter. If there is no word beginning with that letter it will be automatically skipped.
Points are earned based on the letters used in the word - the least commonly used letters are worth more than the more frequently used ones.