Applications are accepted during the issue window, usually three working days. Brokers set their own cut-off time slightly before the exchange deadline.
You need a PAN, a demat account and a bank account linked to UPI. One PAN can submit one retail application per IPO.
An easy example
You select the IPO in your broker app, choose one lot at cut-off price, enter your UPI ID, and approve the mandate request in your UPI app before the deadline.
Important note
The money stays in your account and simply cannot be used until allotment. It is debited only if shares are allotted.
Common mistake
Ignoring the UPI mandate notification. An unapproved mandate means the application is rejected.