We've blogged about why you should do it and how it benefits your own writing. But if you've never been a beta before, you might not be sure how you do it. Depending on how well you know the writer you'll be betaing for, it's a good idea to decide on a few things before you start.
Time FrameIs the writer looking for feedback in the next week or two? Or are they okay waiting a month or longer? Take a glance at your own schedule and see if you'll be able to finish the MS for them in the time frame they're looking for. Of course, we all know life rarely goes as planned. If something comes up and you're taking longer than you wanted in returning the beta, just send an email to let them know you haven't forgotten.
Line Edits or General Feedback
General feedback usually means a writer is looking for an overall feel of the book--if character arcs felt completed, if there were any plot holes, if the pacing was good throughout. Line edits (like copy edits) take a little more time and focus. Line edits usually include word choice, grammar, punctuation, paragraph breaks, out of character dialogue or thoughts, etc.
Once you know what the writer expects from your beta keep a few things in mind.
Most people use the review tab in MS Word to leave comment bubbles for the document. Some people use the highlight function to indicate where they think a paragraph break should go and change font color to red for spelling errors or incomplete sentences. Whatever way you choose to beta, make sure you're consistent throughout the document.
Be HonestBeing a writer, you know how hard it can be hearing that your beautiful MS has some problems. Which might make it hard to point out things you think need improvement. But that's exactly what someone wants from a beta reader. They're looking for ways to make their work better and need your honest opinions. It's up to the writer to decide which advice to take and which to ignore. That being said, it doesn't hurt to point out things you love in the work as well.