good qestion

You have to be lv29, once you reach it you unlock that Feature.

For that go to Ninja - Train and pick the Ninja you want to improve the Stats (c*so be the Main) on, in the bottom you see the tab Cultivate. For each lvl up you get after lvl29 you can improve the Stats on that Ninja 1 lvl

Does that mean that if one level you don't cultivate a ninja, they're stuck with lower stats forever as opposed to having cultivated the ninja levels before?

No, If you forget to cultivate a ninja on one level ,you can still cultivate them ,Its just going to cost more cultivation runes

it doesnt cost more runes if u miss a lvl. each lvl has its own cost and u can gain cultivation lvls based on the ninjas current lvl just like strengthening gear. each lvl costs more than the last and u can choose to do 1 cultivation on a stat or as any as u can afford up to the ninjas limit.

how can you get easy runes?

survival trial and convoy daily mission reward

100000000

Thanks, I'm currently running low on cultivation runes and didn't know where to get more, but ever since I found these forums, I'm able to make my ninja(s) stronger. So, thank you for being helpful. (Sorry, I'm not very good with compliments)