For most projects I work on a fixed scope with an estimated range, based on an hourly rate and the complexity of the work.

In practice that usually looks like:

• A clear statement of work – pages, features, integrations
• An estimated range of hours and a likely cost
• Progress billing at agreed milestones (for example: 30% / 40% / 30%)

For smaller pieces of work, audits, or one-off fixes, I may simply bill on time spent, with a minimum block so nobody’s counting five-minute increments.