According to the roadmap for extra rewards for early miners in September 2019, what should be done to be among the first miners?

Hi yarelydiaz

extra reward is dedicated for holders not early miners
early miners have a natural extra reward due to difficulty stage

Thank you

Who is considered a holder? Anyone who owns some LLG?

If that is the case, how will the amount of reward then be decided?

Well most likely it will depend on the amount of Loligo tokens you hold.More tokens you hold, better the reward.

Is this going to ge some kind of staking system?

No there won't be any staking when it comes to Loligo, Loligo is POW.