Do you have a particular manga in mind? Because first things first, you have to make sure there's no group already translating the manga. Otherwise, it's kind of redundant to have two versions of a translation floating around the web.
Secondly, where is the manga you want to translate? Do you have a physical copy, like from a book or a magazine? If so, you'll have to de-bind it to ensure higher quality pages when you scan it. Plus, it will make it easier to scan. After scanning, you'll have to clean and touch up the pages to make it brighter and get rid of any blurring on the page that may have occurred when you scanned.
If you already have raws that are scanned, great. Then you can just get to translating it normally if you have knowledge of the language. Just make a word doc or something where you can write down your translations. Always proofread it as well to ensure it's readability. Sometimes, a translation can come out sounding unnatural or weird, so it's fine to tweak it to make it make more sense for readers. Provided it still carries the same meaning.
After that, you'll want to clean the raws again to get rid of any text in word bubbles so that you can typeset. If you want to clean away any sfx, it might be trickier to clean, and may involve re-drawing, as sfx often appear on the drawing of the manga panel itself. This includes any small text a character might say that may appear on the background of the panel as opposed to in any word bubble.
Once you're done with that, you can typeset your translations. :D Pick a non-annoying font, or whatever font fits with how the text is written in the original manga, and just type up your translations into your now clean and clear manga page image, whether it's the sfx in the background or the text in the word bubbles. Personally, I like using the font CC Wild Words for normal dialogue.
And congrats! You've translated your first manga page! Just repeat that when doing the rest of the manga.
There's no one way to translate a manga, this is just how I feel like doing it. Also, figure out how you want to do the sfx. Some people write the English sfx above the Japanese ones, some just don't translate it at all, and some erase it completely to insert new English sfx. Do whatever works for you. Translating manga takes a lot of time and dedication, though, so if you can't put in the effort, you shouldn't really do it. Unless you're ok with taking as long as you need.
Does anyone here to know to translate mangas? I wanna translate a manga but I've never done it before so I don't know how...