Can someone tell me how to upload chapters here?

You can use imgur or GoogeDrive or some other place to upload the photos (I’ve found better quality and it being easier on a GoogleDrive but I do it on my phone, there might be different/easier ways on a computer) and you get a shareable link to where you upload them and go to the webtoon you want (or if your starting a new one go to any, it doesn’t matter) and click on the white flag that should appear to the right and click “other” and paste the link and the chapter number (and if it’s new you can add the other information too) and submit it and MangaGo will look at it ^^