I have been involved with fanfiction for 10 years. As a beta, anything I change will be marked and a reason given for why. All of my comments and thoughts will be done in a separate document or added in the email if it's only a couple notes.
I have helped many people throughout the years. Some who are new and just starting out, who are very unsure of themselves. People who speak English as another language, as well as writers who have been around longer than me.
The only thing that I ask of the writer who asks for my services is that they understand I do have a life: Grad School, an active family life, and my own writing. So it may take me awhile to respond, it is not that I have forgotten about you but I just can't attain the time to sit down and go through your work with a fine tooth comb which is the only way I beta.
Also, a must is that I am credited as your beta-reader. If I see that I am not or someone else is, then my service is over effective immediately. All I need is something like, "Beta-ed by Lena204" or however you would like to word it.
Also, if you do ask for my help PLEASE BE SERIOUS! I cannot stress this enough. I like to actually get to know the writer and make sure I know what is wanted/needed. Do not ask for my help and then disappear, because your email will be deleted and I will move on to the next person.