1) Before any Yom Tov that comes out before Shabbos.
2) Set aside a cooked item, like an egg, and a baked item, like a roll, make the bracha and say the declaration you can find in the siddur.
3) The head of the household or his assignee.
4) Everyone who eats in your house.
5) Before Yom Tov begins.
6) You can just leave it aside, no need to eat it. But if you want you may eat it on Shabbos.
7) In order that you should be able to prepare for Shabbos while it’s still Yom Tov.
8) If you live in a community with a rabbi, then don’t worry. He makes an eruv for everybody which you may rely on if you forgot, were stuck and weren’t able to, don’t have any idea how to make an eruv, or lost your eruv.
If not-
If you remember on the first day: Since technically we are keeping two days of Yom Tov because we aren’t sure which is the right day, what you do is you make the eruv with a condition – before you make the declaration say “if today is Yom Tov, then I don’t need an eruv, but if tommorow is Yom Tov [then in truth I’m allowed to make an eruv today, and], then this should be my eruv.” Then continue with the declaration.
If you remember on the second day: You cannot make an eruv anymore. But you need to prepare for Shabbos, so what do you do? If you’re able to, just cook extra of whatever you are cooking for the Yom Tov meal, and leave it on the fire for Shabbos. If you won’t be comfortable with that or it won’t be enough, then you can cook whatever you want as long as you make sure to eat a bit of everything before Yom Tov (and the same idea would apply to any other preparations, i.e. carrying something somewhere). This way you can say it’s technically preparing for Yom Tov. It is a loophole, but completely valid, and in halacha we regard the mitzva of enjoying oneself on Shabbos worth finding loopholes to uphold, and that’s not even taking into account other important considerations, such as shalom bayis.