There isn't one specific way to write a proofreading sample, but there are a few things you should keep in mind. First, your sample should be error-free, of course. Second, since you're trying to show off your proofreading skills, it's a good idea to choose a piece of writing that has some errors in it, so that you can demonstrate how you would fix them. Finally, make sure your writing style is clear and easy to read; remember, you're trying to impress potential clients with your attention to detail and your ability to communicate clearly.
Here are a few places where you can find proofreading samples:
-TheProofreaders.com has a whole section of their website devoted to free proofreading samples.
-There are also some free proofreading samples available on Upwork.com.
-If you have a friend or family member who is willing to let you proofread something they've written, that can be a great way to get a proofreading sample, too.