How Should I Prepare for the Test?

Unlike other exams, the WEPT is not a test for which you can cram. There are, though, a number of ways in which you can prepare yourself for the test.
First, you should assess your performance in previous writing situations—in English classes, in other classes where you have had to produce substantial writing, or in work situations. If your writing has been well received and you have had no reason to lack confidence in your writing, then it is likely (though not guaranteed) that your writing will meet the criteria of the WEPT. It is important, however, that your practice of writing be fairly recent since it is as possible to get rusty in this skill as in any other.
If, on the other hand, you have had difficulty in writing, you should consider what those difficulties have been. Review old pieces of writing and the comments you've received on them. Identify what seems to have been and perhaps continue to be the problem areas. Then do some practice writing and get responses to it before taking the exam.