So first you need to log on fish with your bnet account, you must have a password that contain number and letter "hentai1" will do well. So if it's not the case, change it on bnet.
Then go to this page
https://www.fishbattle.net/freport
(It is in English because it has been translated by Chrome)
1) Category, choose your type of report, most likely "Disconnect"
2) Your G ID, type /g on fish bnet and you will know your number.
3) Party ID, your aka on fish ex: "iFU.hentai"
4) Title, whatever you want, I guess something that summarize the purpose of your report, it will show on the page before being processed.
5) You don't see it (because chrome translated the page) but there is a link, it will allow you to load your screenshot and your replay.
6) This button allows you to link your replay and ss in the "Content" part. you MUST upload a ss or you won't be able to send your report.
7) The link to your replay.
8) Your ss
9) Once it's done click on "send" and that should do it. You can now see your complain waiting to be processed on the report page.
Note that you might want to explain the reason of your complain in the content part. I don't think they have a high level of English nor we do in Korean, so I suggest that you use very simple words (maybe somebody can do a little template for us).
I don't guarantee that it will work. I can't read Korean so if I have misunderstood something, please let me know and I will edit.