Hi! I'm a Sunfish, doing translation/editing work for Touhou doujinshi over at voile.gensokyo.org. It's been just past half a year since I started, so I thought I'd leave some of the lessons I've learned here in the hope that they'll be useful for anyone else interested in helping out with scanlation projects. You can see some of my work here and here. I've still got a long way to go for both jobs, but I'd be happy if any of you find this useful.

Some disclaimers:

1. I am not a native speaker of Japanese and cannot claim to be anywhere close to that level as of yet (though I hope to, someday). I can understand what's going on during a quick read-through, but I need to go a bit more slowly and consult a dictionary frequently for the actual translation work (more on that later).
2. While I can sort of clean gutter shadows, handle levelling and cropping, etc, I haven't yet had to deal with having to redraw massive parts of pages, like someone's hair, or a whole road. In fact, many of the raw uploaders on Voile are nice enough to clean the scans first. (I've been spoiled ><") You might want to ask more experienced editors for help regarding redrawing - I'm still quite inexperienced with Photoshop.

Now then,

Translation

Learning Japanese

Unfortunately, there aren't any quick ways around this step. I started by learning on my own from Tae Kim's Guide to Learning Japanese, before taking formal lessons to improve and iron out a lot of my misconceptions after I'd gotten comfortable with the basics.

If you understand Mandarin, while learning kanji will be much easier, and you can often guess the meaning of words written using kanji without knowing how to read them, a lot of what you know will not carry over. If you don't know how to read it in Japanese, just look it up.

I'd suggest focusing on grammar and sentence structure rather than vocabulary; it's a lot easier to look up a word in a dictionary than it is to figure out what conjugation X does in sentence Y. Speaking of dictionaries...

Use a Japanese dictionary!

While a mobile English-Japanese dictionary may fulfil your needs for the most part, when doing translation work it's often a good idea to consult a Japanese dictionary if you're semi-comfortable with using one - weblio and kotobank are sufficient for most typical words, while netyougo or NicoNicoPedia can be helpful for internet slang. You can also just type [Japanese word] とは or [Japanese word] 意味 into a search bar and see what you get. There are tons of resources out on the Internet; use them. ^^

You're not going to be able to give direct translations for everything.

The Japanese language has a lot of elements that can't be adequately or exactly conveyed in English, and often the sentence structure is the other way round from how it'd be written in English! It's important to provide a faithful translation, but at the same time, trying to capture every word's literal meaning, or trying to maintain the phrasing of a sentence as it was in the original text, will often result in the English translation sounding incredibly awkward. It's okay to deviate slightly from the original text if it helps the result sound better in English.

Editing

First off, this guide shows you pretty much everything you need to get started. I'll just be talking about random stuff that I found useful.

Redrawing, and text-on-picture problems

I don't own a tablet PC, and I can't draw to save my life anyway. This video shows you how to use the Pen tool in Photoshop to redraw an image, and he explains it much better than I could.

I recently worked on a page where it was text superimposed on bookshelves; when these situations arise, you can make things a bit simpler by erasing the original text and adding the English text together with the white stroke first (Layer->Blending Options); this should reduce the amount that you need to redraw. It's lazy and I admit that I do this quite a lot, but between spending twice the time making sure that you can see that fragment of bookshelf between the letters or just whiting it out along with the word stroke, I'd go with the latter. Don't take my word on this, though - different scanlation groups may expect more out of you. Thank you, low standards!

Cloning and such

Cloning backgrounds is a big time sink, and it can't really be avoided most of the time. Sometimes (but often not), you can get a good result by selecting the region of interest, hitting the Del key, and using the Content-Aware fill tool. Other times, when you have a big patterned area to cover up, you might be able to just copy and paste a chunk of the pattern over the text you need to cover.

More often than not, you'll need to zoom to the level where you can see the individual pixels and slowly work your way through. Do your best to make sure that everything fits, and try to use a clone source that doesn't cause your new pattern to stand out. (For example, if you want to clone over an upward gradient, you'll want to use a source from the same horizontal level.)

That's all from me, I guess. If any other translators/editors have anything else they'd like to add, post them here and I'll add them to the OP. Hope some of you found this useful.

PS: Voile is always open for people interested in helping out! There's a list of stuff you can work on here, so if you want to give it a go, just post in the appropriate thread.