The first thing to do to prepare for the writing skills section of the COMPASS test is to go online and review the sample questions You can find these by doing the following -
Now that you know the kinds of questions you will be asked, you can do some review to prepare yourself for the test. Notice that the questions cover "errors in punctuation, grammar, and style."
For punctuation and grammar review, find a writing handbook (available in some libraries, Chemeketa's College Writing Center, some used bookstores). Read through the various sections covering these aspects of writing, including the examples, and, if there are any exercises, do a few of them.
You can do a similar review process online by going to certain websites, such as –
"Style" is a bit harder to pin down because it involves a number of different aspects of writing, such as wordiness, redundancy, word choices (diction), sentence arrangement (syntax), organization, etc. Here, too, handbooks and websites will be useful, but you'll have to look a bit harder for the various sections that will cover these aspects. Here's a good website to get you started:
Once you've identified some of the terminology used on this website, you can return to some of the other websites listed above and search for them or look for them in any printed handbook you've found.