
I know that there are other people explained this but I'll just put what I always did to get free ink.
•permanent ink (don't have expired time)
- watch ads (1 ad got 10 ink)
- complete action (every action give different amount of ink)
•expired ink after some time (easy way)
(go to inbox > gifts or messages to find this)
- claim fortune cookie. it will expired the next monday. at least you can get 300 ink and at most you can get 1000 ink
- join event that said "get free ink". expired time is announce at every event information
- read webtoon that said "read for ink" or sth like that. usually they give 350 ink. expired time is announce at the webtoon info
•some webtoon have "1-tap unlock". you can get about 20% discount to unlock the chapters. make sure you turn it on.

This webtoon is drawn by the same illustrator of "They only Remembered Her"
http://www.mangago.zone/read-manga/truthfully_they_only_remembered_her/
this one breaks my heart but I don't regret it