@Plk_Lesiak I'm not sure if your original question was ever answered, so I'll hop in and see if I can help out
In regards to tools and resources, here's a list of some with small descriptions, in no particular order.
https://owl.purdue.edu/owl/purdue_owl.html Your one-stop-shop for literally anything grammar and writing. Great for both professional and personal work.
https://app.grammarly.com/ I use this tool to help with the proofreading that word/google won't catch. 11/10, would recommend, but don't rely on it alone!
https://www.thesaurus.com/ This site will help with both definitions and, well, anything any thesaurus would help with.
https://www.google.com/drive/ For EVNs, Google Drive is standard. Studios that don't use it are very rare. Touch up with how docs and drive works, if you haven't already
https://lemmasoft.renai.us/forums/index.php The Lemma forums are great for stuff like this. Have questions? Looking for work? Here you go!
https://github.com/ A tool many use for script implementation. Some studios require it as part of their editing pipeline. Good to be familiar with regardless.
https://atom.io/ The best text editor you'll ever use. For real, use it! It even has Ren'Py in it as a recognized language.
Here are some skills that are generally required:
Editing Specific:
Firm grasp of the English language
Understanding of story mechanics/tropes/themes/etc
Firm grasp of character voice
Understanding of how to prioritize changes
Understanding of assets and how the Visual Novel medium works
For example, VNs are a visual medium. You don't need to describe the trees if they're already on screen, unless it's for a specific weight.
Knowing what natural dialogue sounds like, especially for voice-acted projects
All edited scripts, time allowing, should be read out loud before turned into the lead writer/editor/director etc.
Firm grasp of how people act and interact.
General Skills:
Time management
Communication (specifically with writers)
Including the ability to give constructive feedback
Firm teamwork skills/understanding of the chain of command
To touch up on some topics mentioned in this thread:
I do not believe that non-native English speakers cannot or should not be editors for English games. They most certainly can, as long as they have a strong understanding of conversational English. If you've been part of the English speaking world for a long while, even if you didn't initially start with English, you still can edit. The important part is that you must understand how it works from a real-world perspective, both academically and conversationally.
Here's an excellent example: I've known people born outside of the US that moved over in their early twenties. Now they're in their fifties. I am not about to claim they can't edit. Another example: If you grew up in a multicultural/multilingual area, you're generally good to go. The internet is a wonderful place for that. Again, as long as you have a strong grasp of the language both academically and conversationally.
Also, to tackle another more controversial side of the debate I've seen here: VNs are not pure novels, they are not prose. They are specifically written closely to their cousins, the screenplay. In fact, the jargon is the same! We write a VN script. We are going to edit scenes. This jargon comes from both video game and screenplay verbiage.
Remember: in VNs, dialogue is king. Longform prose is not as useful, as VNs have a visual (and audio) element to them that novels do not have. Don't describe the tree in front of us if we can see it - unless it's important to the script in some way. The exception is, of course, for senses that we can't have even in games (smell, touch, taste, and if the VN doesn't have SFX or for accessibility reasons: sound).
Also: it is VERY BAD PRACTICE to have editors only proofread. Generally speaking, if I'm hired on to consult or edit for a studio, and they have editors only for proofreading, I see that as a massive, massive, MASSIVE red flag. I get it, we love the stories we tell. We want them to be our own, right? Well, actually, no. If we're releasing the VN to the public with the expectation that we'll get good reviews or sales (or if we want to make people feel something), then we need to understand that we must be able to kill our darlings. That is to say, when we make a game, it isn't (generally) a solo endeavor. If we have an editor just for proofreading, then we're shooting ourselves in the foot. Of course, if we don't care about how good the game is and just want to make something of our own, go for it! Everyone makes games for different reasons, and I am very fond of (most) any reason to grow the community!
Over the internet, there's a mountain of resources for editing, so I'm sure others can lead you to more useful resources. If you want to sit down and chat with me about editing, I'm always available! I've been working on (and editing) visual novels for half a decade. I also know a lot of bigger editors that you could talk to, should you have the need!
Feel free to catch me on our Discord: https://discord.gg/watercress (My name is WolfGameDev [Watercress]#1785)
Edit: Wanted to make sure I mention that these are just my opinions! We all have different experiences, and I don't want to devalue the other viewpoints. These are my opinions based on my experiences