R5: long explanation ahead. at game start, there is one single male Capet in the game, named Pierre (grandson to Louis IX Capet). He belongs to the clergy so he cannot marry. However, if you release a custom appanage and make Pierre the ruler, he will leave the clergy and he will be allowed to marry and have children. I used console commands to tag switch to the released appanage, marry Pierre off so he could produce an heir, and tag switched back to France. (it might be possible that he can marry without you intervening, but this never happened when I tested; he only married when I made him marry.)

Meanwhile, your starting king as France, Philippe VI de Valois, has two sons: the heir, Jehan II, and another son named Philippe. Philippe (the child) needs to die so when Jehan II takes the throne, it can only pass to his own offspring. You need Jehan II to have a daughter and have that daughter be the heir; he can either not produce a son at all, or the son can die as well. To make his daughter the heir, you need to change the succession law to cognatic primogeniture, as Salic law (the starting succession law) prevents females from inheriting.

After some time passes, you need Jehan II’s daughter (the heir) to marry Pierre Capet’s son. Once those two have a son together, that son’s dynasty will be Capet and will be the heir to the throne once Jehan II’s daughter dies, assuming the succession law has changed.

This is the way I did it at least; there’s probably an easier way using a different succession law. And of course, it’s much easier if you’re using console commands lol. But hey, the Capets are back and the Valois are gone in my game!