So this is for brand new manga never uploaded on this site? Or does this only work for old manga that hasn't been updated in a while
yeah it's for new series that aren't on the site yet. Just go on any series and click the gray flag on the left side of your browser and follow the steps in the link.
Before the site shutdown, you had to do the above method to add a new series on the site, and upload chapters to already existing series. But they updated the site after it came back on to include an upload chapter button on existing manga now. So you don't have to do the above method to upload chapters anymore, but you still have to do it if you want to upload a new series.
How do I upload a manga that's not been uploaded yet?