A while ago I offered to be a beta reader for a friend of a friend. I had given feedback on several books though these consisted of haphazard replies rather than a streamlined critique. In any event I thought I was ready to tackle a formal beta reading. Note to self, no matter how much planning goes into it writing the first draft is not a good time to beta read. The reason why I offered is because I think feedback prior to publication is valuable. Be prepared to set aside a bit of time, it is not the same as reading for the sake of enjoyment. You get to answer questions, and make comments at the end so it is worth taking notes.
Beta reading is best done with a genre that interests you, find out the basics such as the word count, either a blurb or synopsis, and a few basic details like the first chapter. After all this is a story you are going to become familiar with and it helps if you like the style. To be honest I was not sure what I had gotten myself into, it was a big commitment and I was not sure how much I would pick up or be able to comment on.
My greatest concern from the moment I began was how fair I could be, and would the comments be amicably received. Not everyone wants to hear something constructive, I use that word because I think there is a distinct difference between constructive and negative feedback. Constructive feedback is about providing a balance and an explanation. I find it just as important to highlight both the strengths and weaknesses to create a clearer picture for the author.
Honesty is best but if something really does not work explain why. It is a fantastic effort for anyone to complete a manuscript even in draft stage. A great many hours have been spent, and it is worth holding onto that thought when providing comments. Also, be honest about what you can and cannot do. I shy away from grammar, but my strength is the overall story and its rhythm. I have spent too much time away from writing my first draft, though I am thankful to assist as others have done for me.