Keep using the grammar rule til it's too familiar, then go to the next rule.

My native language has, thus far, managed to conserve many of its endings. We covered them in the school; maybe there was a couple of hours used on the fancy foreign names they have, but aside from that, no more formal teaching.

You should definitely know that the endings exist and have read through them, but I recommend exposure to the language as the primary way to learn them.

Exposure

As a native speaker, I do not have a long list of rules memorized. As a learner of other languages, I also do not have such lists, aside from the -ons, -ez, -ent in French (thanks to a couple of years at school). Rather, one gets a feel for the language after sufficient exposure and use. So: Read and listen. Start writing and speaking when you can, too.

Note that the spoken language might very well have different or poorer endings than the written language. This is the case with Finnish, at least. So take care when listening.

Exercises

You might want to use Clozemaster or some other learning tool that requires you to get the endings precisely correct. Duolingo is lax on diacritics and non-English letters (in Finnish, at least, a and ä, for example, are completely different letters), but maybe these are not a problem for your language. I recommend something that is not lax about such matters.

When you are advanced, start checking the grammar again

After you can read texts and follow them, and maybe even follow speech, you might want to start checking grammar again. The right point to check it is when you are surprised by something.

Maybe memorize some prototypical or exceptional examples

It might speed you up to memorize a couple of words whose patterns many others use. If there are highly irregular but common words, but only a few of them, maybe learn them by heart, too.

There will be some holes

This approach will mean that you will mostly learn those tenses that are actually used in wherever you get your exposure from. If you do not enjoy old or fancy literature, you might never have seen some rarer endings. But then, their value to you is presumably not too great, either. It might be the natives do not know them, for that matter, at least actively.

But if this is not acceptable to you, you might either look into old or formal writing, or alternatively learn grammar from a sufficiently exhaustive book or other source. But this is certainly very advanced study, similar to learning rare English words that have more commonplace alternatives.