I want to add a new manga work to a series

You can add a work to a manga series on the desktop and mobile versions of pixiv.
It cannot be done from the app.
The procedures are as follows:

＜On desktop＞

1. Please click "Post your work" and then the "Series" drop-down menu from the posting page. You'll be able to choose either a series you've already made or "Add series" to make a new one.
   
   
   
2. After filling in the required details and then clicking "Post", you'll be able to simultaneously post the work and add it to a series.
   
   

＜On mobile＞

1. Please tap "↑ Post" and then "Manga".
2. Next, tap "Other settings" and then the "Series" dropdown menu. You'll be able to chose either a series you've already made or "Add series" to make a new one.
   
   
3. After filling in the required details and then tapping "Post", you'll be able to simultaneously post the work and add it to a series.
   
   Please check out this page if you'd like to change the display order after posting the work to a series.