In the old Soviet Union it was tried and it not only failed but faith is very strong there today. In protestant North West Europe most counties have state churches and religion is fast heading for extinction here if current trends continue. Correlation is not always causation. But......

Iceland is almost 100 percent atheist on the whole, and I think everyone under thirty is atheist.

We need some land to put out nation.

Do you really think it's a good idea to exclude from this new nation of yours anyone who is not an agnostic or atheist? On what moral grounds? Attempts of this kind have been made by atheistic communist regimes, and failed dismally.
You ask what conditions would be needed. You'd have to employ secret investigators to sniff out any believers who secretly enter this nation, and you'd have to have punitive laws to deal with them, lest others join them. Is that the kind of society you want?

Very simple. In the US we reestablish the boundary lines for the old Confederacy, build a big wall around it, and if anyone commits hate crimes and bigotry including in the name of what's his name, we banish them to there.

It’s been tried many times over. Read “State Atheism” in Wikipedia. It was attempted in Mexico in fact. All attempts to force people to change their natures have failed. People are primary and governments have to follow suite.

Well that would take all the fun out of not being one of the sheep!

Put all the religious nuts in prison then tell them they can only be released if they never speak about deity of demigod again.

The trick is to create a nation where religion is no longer relevant. That requires you/us to build a nation that values every citizen and provides them with the opportunity to 'be the best that they can be'. It won't happen under the present economic system. ?

Turkey tried something like that under Atatuerk see Wikipedia
Secularism in Turkey defines the relationship between religion and state in the country of Turkey. Secularism (or laïcité was first introduced with the 1928 amendment of the Constitution of 1924, which removed the provision declaring that the "Religion of the State is Islam", and with the later reforms of Turkey's first president Mustafa Kemal Atatürk, which set the administrative and political requirements to create a modern, democratic, secular state, aligned with Kemalism.

Nine years after its introduction, laïcité was explicitly stated in the second article of the then Turkish constitution on February 5, 1937. The current Constitution of 1982 neither recognizes an official religion nor promotes any. [1]

This is as far as it went.

Some of the Scandinavian countries have extremely low numbers of religious people. It'll happen eventually.

you should read Yuval Harari's "Sapiens". I think it would help answer your question.

it cannot be done and it never will be done. the soviets tried. look how THAT worked out.

g