When you add story to want to read/reading/already read list there is always option to add your own tags and use your own tags you've created before, rather than use the popular tags (made by other people) for the story.
Click any story, click on want to read (or the others), a pop up will appear showing option to give star rating, subscribe to the manga, add tags, tags I've used before (tags you've added for other stories previously will be here. But if you haven't done so previously, this part will be empty), popular tags, and comment (not functioning right now tho).
Just enter which ever tag you want and click enter key on your keyboard. It will be added. Once done, click submit at the bottom right corner. Now whenever you check you reading lists, if on mobile, it will show all your tags and number of stories with that tag at bottom. Or if on desktop, it will show them at the right side I believe.
if you're on an android device, you may be facing a glitch where when you hit enter to add the tag, it sends you down to the comment section.
if this is the case, it personally works for me if i use the firefox browser, or if i use the third party keyboard from the play store called "Fonts Keyboard".
if you're just have trouble finding where to add the tags, the comment above will be helpful!

Okay, I've searched for how to do it long enough...
HOW TF DO I MAKE MY OWN TAGS?!?