How common is it for lay people and for monks both to attain higher jhanas like sixth, seventh and the eighth ?
No monk or layperson would post their most noble attainments on a website. Even if somebody goes to them and asks personally and secretly, they wouldn't tell.
How easy/hard it is?
It's not common. It's very rare. Among lay people, it's even more rare. My guess among monks is 1 in 100,000 and among lay people 1 in 10,000,000. It is rare because they don't put in enough effort, they don't have the capacity, don't have a suitable background, don't have a suitable teacher, etc. Let alone achieve it most people in the world have never heard of the word Jhana. (more than 90%). Even among those who know about Jhana, like 90% of people are just living their normal lives. Even among people who try to achieve any Jhana, like 90% just give up or don't achieve it. That's why it's rare.
Have you entered any of the Jhana states yourself?
No. I say this because I haven't. If I had, I wouldn't say it like this or be even typing stuff on the internet like this. (Not angry or blaming, but just saying).
If yes, what was your method?
Even though I haven't achieved it yet, I do try. A collection of methods. Anapanasati, Meththa, one from 10 main Kasina.