@Ixayoi
Save the chapter's pages as individual .png files.
Number each page's image file in order (00.png (optional, for your group intro page if you have one), 01.png, 02.png, etc).
Put all the pages for the chapter in one folder and compress them into a .zip archive (ex. trinity_ch90.zip) I don't think the .zip file name actually matters, except to help you keep track of things.
Go to https://mangadex.org/upload/
It should be self-explanatory from there.
Edit: It seems like you uploaded a chapter as /a/. That's fine, if you're collaborating on /a/, but based on your comment I'm not sure that's the case. If you're a group of friends who didn't meet/don't communicate through /a/, then it would be more appropriate to upload as "no group" instead, or create your own group.