You indicate your interest. Also start funding the account. So you fund 7000 for 100 shares at 70. Then they announce the price is 75. You must change the order to fit the funds or have extra money. Of course you can opt out or change the quantity anyways. But I think in both the cases I actually went ahead (Reddit and figma), I had to reconfirm the quantity. You can't be away from the network.
Pretty hard to get in on a hot IPO, though, as the shares will go to their favorite clients (those with the most money).
You can submit an order via your broker (depending on the broker and your account) to the auction itself on the exchange once it’s in the quotation phase.
Not that I would advocate for that. Sometimes it’s better to buy in the secondary market once things settle a bit.
But otherwise yes it goes as described above.