How do you get a unguided timed entry into st. peter's basilica

You don't.

Everyone who goes into St. Peter's waits in line for the security check. There is no skipping that line.

Your best bet is to get there early. It opens at 7am every morning except Wednesday.

It is free to enter.

New for this year you can buy an audioguide that includes access to a shorter security queue. In March, the only slowdown in that queue was the nun in front of me who slightly struggled taking off her jacket. https://www.basilicasanpietro.va/en/products