Where a retail offer is available, retail investors can typically apply for shares through a designated platform or intermediary (such as IBKR). Generally, this means the investor must already have a brokerage account with the intermediary or open one prior to applying for the IPO shares.
The details of how to apply will be set out in the prospectus and any accompanying offer documentation.
It is important to read the prospectus and/or any other offering documents carefully before deciding to invest, paying particular attention to the sections on risk factors and the terms and conditions of the offer.