It's very, very, very difficult.
I struggled with writing in my younger years a lot. I worked for a computer company in California that made me take some writing classes. It helped.
You know what else helps? Writing reviews...
My earlier reviews were bad and poorly written. I still think I struggle with writing but like anything, if you think you're bad and work your ass off at it, you can generally do alright.
Even now, though, I read my reviews a few times over and find corrections. I usually read through my reviews 3-4 times before I actually post it, and even then I'm sure there are still errors. I know part of my problem is that I'm a fast typist. Sometimes I think one word but type another, like I'm out of sync or something.
Most authors have editors and I'm sure most magazines and larger websites have them as well. For us, well, it's yourself and your readers if you're lucky.