I don't have a lot of experience as a beta, but I do have a lot of writing experience. I have taken fics before and edited them with the author's consent, but just for myself, as practice. I tend to be very critical, but I am not mean about it. Accuracy and factuality are important to me, realism (even in a sci-fi or AU) is my key for a successful and enjoyable read.
The first thing I do when editing is go through the entire portion and examine the consistency, flow, and dynamics. From there, I read it a second time and can move on to correcting spelling, grammar and the technicalities. I will try to be quick, but thorough, when editing, but I may need some friendly nudges to stay on top of things.
1. Please, label for trigger warnings. If I know it's coming I can deal with it and won't have any issues.
2. I will be as courteous to you as I can, please respect that and do the same for me.
3. You don't have to take any of my edits as requirements, they are simply suggestions to help better your writing and make it more enjoyable for readers. That doesn't mean they are free of errors. No hard feelings if you don't like my changes.
4. I understand that rl can get in the way and sometimes the unexpected happens, but please communicate with me so that I'm not left hanging.