The booking flow at 18 is no different from booking at any other age. The mechanical difference is at the front desk, where the agent checks the age on the photo ID against the property's minimum. Properties listed here have set their minimum at 18, so the reservation goes through and is honored on arrival.
Independent boutique hotels and budget chain franchises (Motel 6, Super 8, Days Inn, La Quinta, Red Roof) are the most reliable bookings. Mid-scale brands like Hyatt Place, Holiday Inn Express, and Comfort Inn vary by property. Sonder and Pod Hotels are platform-wide 18+. Hilton, Marriott, and Drury default to 21+ at most properties.
What you'll need: a government-issued photo ID matching the reservation name, and a credit or debit card in your own name for the deposit hold ($50–$200 per night). A parent's card will not work even with written authorization at most 18+ properties.